the answer to overclocking is DO IT.
but you want the BEST VIDEO CARD AVAILABLE, (personally I like price\proformance)
my vote on the GeForce 7900GT , if you find a good one it can OC higher than a GTX I'm sure.
BUT, OVERCLOCKING IS THE WAY TO GO!!!!
With my XP-90-my Opteron 144 right now is OC'd to 2.9ghz BF2 temps of 47-49max (thats not even it's max, waiting for my RAM to finish RMA and mine's been tested stable 3.1ghz)
$200 (Opty 144) vs $800 (Opty 154)
Opterons are the best though, If you have money I'd go for a Opteron 170 Dual core (shop around for good stepping)
RAM=2gb OCZ (find a good model, run you around 170-200 for a nice pair if you shop) I also hear good reviews about Mushkin's REDLINE set (basically preference to company)
Go with 2gb unless you want 1:1 at 300+ HTT in which case you'd wanna try to find some OCZ PC4000 Gold VX or OCZ Rev.2 1gb set (both can go over 300+ if you find don't get a DUD)
Motherboard=depends if you want SLI or not, PCI-E dual 16x on the A8N32-SLI is slightly better than dual 8x, maybe around 5%.. ?? (correct me if I'm wrong anyone) but not THAT much, anyway.. if you want to overclock though, DFI motherboards are a MUST.. of which the best of them would be the DFI Lanparty UT SLI-DR Expert
Hard Drive= Go for a 75gb Raptor as your boot drive (150gb if you have cash to spend, and 2x150gb if you have TONS of cash to blow) and a Large GB seccondary (250-500gb) /w 16mb Cach
If you don't wanna go with the raptor the next best puppy is the 500gb Hitachi drive, Model 7k500 check out some reviews
Case= Whatever you get, make sure it has plenty of airflow (makes a huge difference to preformance)
PSU=OCZ 600w Powerstream, or Enermax Liberty (theres other stuff, and if you don't want to spend tons the OCZ 520w powerstream is pleanty fine even for SLI but no reason to go with an unknown when companies like this give such good products)
BTW: If looks are Important OCZ's Modstream model's are nice,
check out neweggs reviews for stuff on anything you buy, they're usually accurate and helpfull
Cliffs:
HIGH END Video: 7900GT
Opteron 170
MB:ASUS A8N32-SLI for SLI non OC
DFI Lanparty UT SLI-DR Expert
OCZ 2gb or Mushkin Redline
150gb Raptor or 7k500
CASE:AIRFLOW!!
600w OCZ Powerstream
MEDIUM... END.. lol
7600 GT
Opteron 144
Dont Skimp on the Motherboard go with High end
1gb PC4000 Gold VX
7200rpm 16mb Cach HD
Case: See High end recomendation...

OCZ 520w Powerstream