Okay folks, here's my dilemma.  I have a Dell XPS from mid-2006 with Vista that came with a Pentium D 2.8Ghz, 1GB 667Mhz ram, some Dell BTX factor board, and a ATI Xpress 300 card.  I wanted to do some gaming, and I upgraded the RAM to 3GB and added an 8800GTS card.  I'm assuming that my 2.8ghz Pentium D is SERIOUSLY bottlenecking my card because in games like Unreal Tournament 3, I barely get a steady 40+ FPS and I've seen benchmarks with this same card where the FPS never drops below 60-70 range.  I have not even tried running Crysis.

So I've decided to build a new system with a gigabye ga-p35-dl board and either an e8400 or e6750.  I will probably end up going with whichever processor I find cheaper on Newegg because yes, I have a VERY tight budget.  So mainly, my question is that is this upgrade worth it and will I see a substantial increase in my FPS?  I have a top of the line card.  I'm willing to spend $200 on a new processor, $100 for the mobo I mentioned, and a $100 new case.  

Also, will my current hard drive work in the new system?  It is serial ATA.  I know the Dell Vista install is OEM and locked to that board.  However, I'm hoping I can pop the drive in the new system, boot up into Vista, and have it ask me to RE-activate and then I can just enter in a new product key from a new bought copy of Windows Vista home without having to format and install.  I really dont want to have to format it and install, but I will do that if i have to.  As for the PSU, I already snatched a 550watt antec psu which should be plenty I'm hoping for the new system with just the 8800gts, 1 hd and dvd drive and a couple fans.  Thank you for help!

Lastly, I have a question about the cpu fan/heatsink.  My dad had an older system from two years ago which had the motherboard die suddenly (tried replacing everything else including memory and psu and still would not boot) which he gave me to mess around with.  Anyways, I can put the whole system together, but damn the stock Intel heatsink/fan is such a pain in the butt to install.  I swear I almost bent the board trying to get it to pop into the holes.  Are all heatsink/fan products just as hard to install?  I've heard alot of people recommended artic cooler pro or something like that.  Are those any easier to install?

Yes, your cpu is bottlenecking your vidcard, so you should see huge improvements if you get the e8400.
 
As far as I know, from experience, you can not just put the old HD into a new build if it is locked to the board. I tried, but it didn't work.

what if i format the hard drive?  It should work then, no?

http://www.ncixus.com/products/277 [...] 400/Intel/ HUGE PERFORMANCE INCREASE! This is coming from someone who went from a P3 800MHz + 32mb vid card and 256mb RAM to a Q6600, 2gb ram and HD2600XT 512MB, hell my VRAM is more than my old RAM.

 
Sure it will. You should be able to use both the hard disk and optical drive from your Dell system. Have you checked to make sure you can't put a C2D in that Dell box though? If you could you'd save yourself a considerable amount of cash.

Usually the Dell case uses the micro-ATX motherboard so if he's getting the reg sized ATX mobo, it won't fit.

Yeah, it turns out the C2D have different voltage.  A few people on the Dell forums actually bought the C2D's and put them in.  Same socket and everything, but it would not boot.

dell doesnt use matx it uses btx from every one ive seen in the past 2 years.  You can easily tell if yours is a btx case if the heatsink sits near the front of the case with a big shroud that goes to the front of the case

how about the e6750?

What about it? With the e8400 out, it's probably better not to get the e6750.
 
@xnamerxx
You're right, my mistake. I don't own a Dell, never did.

None of the boards in my range support the e8400 out of the box.  They all require a bios update.  I don't have an extra processor lying around to fire up the board in order to apply the bios update, so I'm kind of stuck with the e6750...

All LGA775 boards that support C2D should support your current processor. Stick it in, flash the BIOS, replace with E8400.

The gigabyte GA-P35-DS3L rev 2.0 will support the E8xxx processors with the F8a bios.   If you order where they have fresh stock, you should get a rev. 2.0 board with current bios.  Your current P4 will also work in the event you need to update the bios first.  No sense to me to get the E6750 when you can get a E8400 for near the same price.
 
For the os, if you got a vista oem cd from dell, you will likely be successful in using it on a new mobo.   Activation will invite you to call microsoft.   Just explain that the mobo was bad, and you are replacing it.  They will ask you if this copy of the os is used on anyother pc, just tell them the truth, just say no.   If you don't have the cd, but do have the many digit id code, then see if you can borrow any copy of vista to use.  They are all the same, it is the id code that tells what version you have.   Otherwise, vista is discounted for students if you have one in your household.
  If you have data you want to keep, back it up to an external device.  You should be doing this for anything you care about anyway.   You could try a repair install  of vista which would preserve your files and settings.   Knowing that dell will include "bloatware" in it's systems, I would advise cleaning it up with a clean install.
 
I assume the 3gb of ram is ddr2, otherwise you need to add it to your budget.
 
The stock fan is best mounted  with the mobo outside of the case.  You need to be able to look at the back to be certain that all 4 pushpins are all the way inserted.  If there is a trick to it, it is to push on the two diagonal pins at the same time.   Many oem coolers use the same mounting.   Some will use a back plate which is more secure.  They also is mounted with the mobo outside of the case.
 
The dell btx case may not work, but check it out in case it has atx mounting holes also.  Sometimes cases are built to handle multiple types of boards.
 
---good luck---

thanks for all the replies, especially geofelt.  I know the dell btx case won't work because atx boards open up from the left side and the board is mounted on the right side.  Well the btx case opens from the RIGHT side instead and the board is mounted on the left.  Completely opposite.  Thanks for your advice though.  Hopefully the new hsf won't be difficult to install.  Yes the 3gb ram (2x512mb and 2x1gb sticks)is ddr2 pc5400 667mhz so I think it should work fine, if not I could probably trade it in at a local computer store.  Actually, the dell hard drive has a fresh install of vista.  the original hd the system came with died on me a month ago and so dell shipped me a replacement that was completely empty, and i had to install vista and all my software and games from scratch.  I'm just thinking about doing a fresh install so avoid any headaches in case it causes stability problems or anything similar to that with the current installation.

You have to do a fresh install with a non-OEM copy anyways. It will bluescreen on boot and cannot be repaired because it bluescreens when booting from OEM cd too.

i dont know about bluescreen when booting from an old vista hard drive from another computer, but i know you CAN boot from an OEM disc and install.  I've done it before.  I had vista on a notebook that broke down and dell wanted to charge me $200 to fix it. anyways, i took the vista disc that came with it and installed it on a different computer and it worked perfectly. i do use that computer, and i could use that disc on this new system too, but I dont want microsoft to send me to jail.