After reading the "who can list the greatest computer??" thread it's interesting to think about how big an upgrade my current computer is over the first computer I owned 20 years ago, a commodore 128.
 
Current PC: 3.2ghz 64 bit core 2 duo
C128: 6510 8-bit CPU @ 1.023 MHz
New Vs Old: 25,600 times
20 years from now: 81,920Ghz equivalent (maybe  3.2Ghz with 51,200 cores)
 
Current PC: 2 gigs of ram
C128: 128kb
New Vs Old: 15,625 times
20 years from now: 31 terrabytes
 
Current PC: 320 gig hd
C128: 170kb drive
New Vs Old: 1,882,352 times
20 Years from now: 600,000 terrabytes
 
Current PC: 1920x1200(2,304,000 pixels) Display
C128: 320x200 (64,000 pixels) Display
New Vs Old: 36 times
20 Years from now: around 12,000x8,000 or 82,944,000 pixels  
 
Current PC: 113 Keys
C128: 90 Keys
New Vs Old: 1.26 times
20 Years from now: 142 keys
 
Current PC: 700 watts
C128: 40 watts
New Vs. Old 17.5 times
20 Years from now: 12,250 watts
 
Please check my math, my calculator doesn't use commas so it was somewhat hard to read with all those zeros

I want to say that some of the things you listed can't be possible, but seeing as how those are famous last words, I'll just say that with Moore's Law effectively shot to hell, I know absolutely nothing and just hope that time and unforseen circumstance allows me to live into my late 30's to see the advancements and come back, dig up this post and either say yea or nat.

I think that this is an interesting take on the future of comupters, but this industry is a fickle and unpredictable one.  You know that little quip about "never needing more than 640K of RAM" - that wasn't quite so true, now was it?  Having said that the industry is unpredictable, you can't really fit its progression to a linear curve as you have - I'm sure that the industry would deviate from the line all over the place.
 
My prediction: in the next twenty years, society will push for greater integration between computers and their users - us.  An intersting project codenamed "Digital Angel" sprang up about six years ago, with the intent of implanting people with GPS tracking microchips.  I don't know what happened to this project, but to me, its some kind of precursor to the future of society.

. o O (I wonder if I still be around over the next 20 years)
 

its neat to try and consider what 20 years might have in store, and I think it'll be a little more than just straight linear interpolation of statistics.
 
Also consider the acceleration of technology.... I think it is tapering off as far as raw numbers are concerned.  Raw CPU speed has tapered a little while "different" computing bottlenecks are being addressed.
 
Other concerns I think will go in a different direction.  Our power demand has gone up as a direct result of greater computational power and speed... but I'll hedge my bets on greater efficiency, total power consumption goes down, or the watts per CPS actually goes down, in the next 20 yrs.  
 
I don't believe keys will increase. I think something phenomonal will replace the keyboard as an input device.
 
I'm excited

I can find most of that quite plausible. However what will prob happen before 20years is up is the PC architechture will TOTALLY change.
Hardware and software changes will likely mean a slow growth of certain numbers like cpu, significant speed gains will be made in more instructions per cycle, and more importantly, more efficient programming requiring less resources (i hope, unlike vista)
 
If we use the old engine analogy... in the 70s BMW had 1.5L turbos with around 1000KW.
Today my 5.7L v8 struggles to get 300KW. Even the 1000KW 3.xL R34 that blew my mind in comparison is pissy.

 
 
11520x7200 (assuming the "multiply by 6" method is used).
 
Still though, I wouldn't mind an 82 megapixel monitor. I might be able to run Wolfenstein 3D at that resolution (using quad Sli, minimum effects enabled and at a frame rate of 25fps).

Actually, I am more amazed at how little the actual "user experience" has changed in the past twenty or more years. Let's face it, we're all using fancy-ass, super-fast Xerox PARCs with vintage 1970s technology: Keyboards, mice and windowing-OS's are almost thirty years old. If you go back thirty years before that, computers took up entire buildings and were programmed by hard-wiring!
 
When I was a kid I figured by now, I'd be having philosophical discussions with my own HAL 9000, not still banging on a QWERTY keyboard designed to keep speedtypists from jamming the keys on manual typewriters in 1868!!! Where are the universal speech inputs? Where are the eye-focal pointers? Where are the neural interfaces? Hello?  
 
Even the speed itself of the "user experience" hasn't changed much since my Mac Plus. Even though my CPU is a few zillion times faster than a 68000, Word still takes half a minute to repaginate my manuscript... but it does it in 32 bit color... which is such a help when I'm writing a report... DUH! And my Mac Plus booted up in a few seconds... try that now with XP. So today I can Gigaflip and Gigaflop. Big frakkin' deal.  
 
I'm massively unimpressed by today's computing experience. The vested interests embodied in Satan Gates and his Satyr Henchman Jobs have effectively frozen the computer "user experience" to about 1980 when they first got into the business. They have done more to hold us back than anything else by distracting us with the mad "performance" game which looks great on charts but doesn't mean tiddly-squat to the average computer user.

 
But today's mice have lasers... LASERS! That has to count for something, right?  
 

 
 
It's getting there; you can buy IR trackers for flight sims and weird funky ass keyboards which aren't QWERTY. I remember seeing on the news a story about a girl with cerebral palsy who was able to communicate with people by looking at a certain letter on a sheet of the alphabet which was hooked up to a computer in front of her. When she looked at a letter the computer would speak it out for her, which is pretty damn cool actually.
 
I remember another peripheral for the PC (it was announced but I can't remember if it ever saw the light of day), which was basically a blood pressure monitor for your finger. The object of the "game" was pass certain challenges by keeping your blood pressure, and thus stress levels, down. I think it was for a stress management app.
 
Also, vehicle manufacturers are testing sensors that look at your eyes and can tell whether you are drowsy or not. I'm sure some entrepreneur will come up with a way to retro fit it to games.
 
Edit: Spelling errors

 
You make some interesting, and not entirely untruthful, observations. Certainly my computer is much faster now than at any time in the past for office tasks (Core 2 duo at 3.4Ghz, 2 gigs of ram - so not only word loads instantaneously, but pdfs in acrobat do as well). Another big revolution has been the capacity to handle video, so you get not only youtube, but can watch tv episodes via bittorrent. But the user experience hasn't changed that much.
 
To me what is most amazing though is the advance in games. Playing something like Far Cry on a home PC is incomprehensible given the state of games on computers like the Commodore 64.

Dude, you are an immense dork with way too much time on your hands.
 
Sadly, as I sit in the early morning hours avoiding my MCSE studies I can relate all too well to your most excellent post.  
 
Since I, too, am old school having owned a TI-99/4A (my how I envied the Commodore 64 owners) and a Mac Plus in the early days I'm afraid you negletected two other very important aspects:
 
1) How about a calculation about the real estate?  My Mac plus and it's whopping 1 megabyte of memory and 8 mhz processor required 1.5 (?) or so cubic feet, youd have to figure that your 20 year future computer would be about the size of a single dice (die?) .... which may make plugging in monitors a bit tough.
 
2) The other thing you neglected was, of course, price. That mac plus with no HD and two floppy drives (one internal and a second external) set me back $2400 bucks.  Adjusted for inflation, if that were to be calculated in $ per teraflops, for example, that future computer would have to be free or purchasable with pocket change at best.
 
The external hard drive I later purchased for that computer was $600 bucks, 15 or pounds or so, and probably 1/3 cf in size to get me a quite thrilling 20meg.  I have a 1gb SD card (another for misseur? Is wafer thin) for my camera that I paid $25 bucks for after rebate that weighs literally nothing by way of statistical comparison. That's 52 times the storage for about 4% of the original cost and literally 0 size/weight by comparison.
 
Heck, at that rate, 20 years ago I should have been able to predict that I'd eventually be able to get a computing/gaming/database device that would weigh nothing, have some form of built in networking, fit in the palm of my hand, have many times the processing power of that desktop computer, be able to play games - perhaps even in color, and that it would be FREE!!  
 
er... uh... wait, that sounds a lot like my cell phone.
 
Now before you young whipper snappers and fanboys mock me too hard, keep in mind that you too will one day grow old and your stories of of your present day radical gaming machines will seem as ludicrous to some youngster as my thrills with Donkey Kong or my tales of the abject awe inspired by the first time I played Dragon's Lair. 8)

 
         
 
OLED keyboards... hopefully context sensative that switch with application focus. That's what I'm thinking will be the next move in HCI.

Dragon's Lair was the beginning of my ongoing objection over graphics excellence vs. gameplay.
I too was impressed at the visuals the first time I saw someone playing Dragon's Lair. But $3 worth of game tokens later I was bored with the game. It was more like a memory/reflex testing device than a fun video game.
(Scenarios were identical each time. You lived or died by moving a certain direction at the exactly correct time. Often this was arbitrary. Move to the left in one scene, you died. Only way to figure that out was to play it once, and get killed. Then restart, and next time, you had to remember to go to the right and not the left.)
Anyway, I quickly went back to playing Joust, Spyhunter and Miss Pac Man. The visuals were nowhere as good but those games were a whole lot more fun!

The sad part is that even if your predictions come true, it'll still take windows 30 seconds or more to load.

I just cant believe that you have the time to calculate all those predictions,
i dont think pc will get any faster, instead the software will become simplier to process and the system will appear to be running faster >      
haha thought bout that....

If I had to guess, computer technology will continue to grow at an exponential rate (i.e. technology becomming faster at a faster rate) until there is one new innovation related to computer technology that changes everything else. Rinse and repeat.
 
Think about it like this. Cavemen lived in darkness until fire was found. Fire was integrated into everything they owned and evolved until something better came along (lightbulbs). Now that they have been out for a while, lighting technology has improved (different variations of lightbulbs) as well as increasing effiency.
 
Although this is somewhat a generalized statement I believe this is the way things are.
 
Creation -> Improvement -> Innovation.

This is one of the most open questions ever; the most approximate answer is "WHO KNOWS?!?!?". IT's much easier to guess what will the world be like after 20 years, will this humanity still exist or not, what will the sea level be etc. Especially now that the silicon process is slowly facing a dead end, nobody really knows, nowever, we all notice some slowdown in the progression; there is no more doubling the frequency (& performance) every year like 6-7 years ago and even though Intel and AMD are pushing more than ever,the greatest chip in 3 years, Core2, only managed ~30%+ compared to 1 year old AMD designs.

It's also interesting to think about what new technologies have become mainstream between my C128 and current pc:
 
The Mouse and GUI
3D graphics
Laser and Bubble Jet printers
Hard drives
Optical drives
LCD displays
Viruses
 
And the things that have dropped from the mainstream:
 
A Basic programming prompt at bootup
Dot Matrix and Daisy Wheel Printers
Tape drives
Floppy Drives (also flipping disks)
Dip Switches
Monochrome displays