939 upgrade

For the price you may as well just go for a cheap am2 + or intel 775 setup as there's no point in upgrading that further. The performance increase will be far better than just getting the opteron. Also 2 gigs of ram is always good.

As far as the CPU goes your wasting money trying to upgrade. There may be a few dual core Opterons floating around, but they wont provide much of a boost. Your best bet for that system is to OC your 3800+ and get another year-year and a half out of it. I would put that $200 towards a new GPU. HD4850 is making a big splash in this segment, but you could also grab an 8800GT(S) and another GB of ram.

HI
"I want the biggest performance increase" you are asking very vague question, performance to do what? browse the web, email, lol, maybe you play games? which then?

i would get another 1 gig of ram
also if you are gamer(and depending what games you wana play), new vig card, but i believe 939 only support AGP (except few MB's) so that wouldn't be future proof but mostly waste of money if you plan to get newer system in year or so!
amd x2 4400, 4800 looks like the only place to buy them is eBay, but they go over 100$

so in any situation you need to get system to total of 2GB of ram
and then depending on what you do,
new vid card or buy new (0r used) CPU(4800 or atleast 4400) of ebay and that will be $200

He has an SLI mobo, PCI-E FTW

My bad there about AGP comment

but still it all depends on the reason for upgrade
+1gb ram and video card if you have trouble playing games and want to get more FPS
+1gb and CPU if you work with CPU intensive programs(+everything non GPS related)

or you can just buy new AM2 motherboard+ram+CPU, since current parts on market cost less then for older system (DDR vs DDRII for example)
but that would still put you in same boat as with old system if you wanted to get better gaming performance, you get boost from new CPU and ram but not as much as from new GPU

Only one thing though, the mobo you suggested can be 50/50 right out of the box, and also it has a lot of compatibility problems with different RAM brand/types etc... I built many office systems with that board, and like I said, without any exageration, it's almost a 50/50 chance of either getting DOA or RAM compatibility issues. The problems often stems from really old versions of the BIOS being shipped with the board itself, but unless you have different RAM types/Brands lying around to test it out, you can get stuck quickly.

My 2Cents.

I say for your money, you could even go with a cheap AM2 rig like some already suggested. Take it from someone who tried cutting corners already, by the time you buy all the old stuff, you might as well go to newer hardware that you have upgrade paths with.