Alright, first off, welcome to Tech Forums.
Best computer you could probably get with the money you have:
AMD Opteron 170 (or AMD Athlon 64 X2 4200+/4400+)
Geforce 7900 GT
Antec TruePower 2.0 550Watt
G.Skill dual Channel 2x1GB Extreme
DFI Lanparty nf4 Ultra-D
You're gonna need hard drives, optical drives, and a case.
Before I submit this. I feel sort of obligated to mention Conroe to you. Conroe is going to be the next processor coming out from Intel on July 23rd. I don't want to sway too much off topic, but it's going to be amazing. Use your search button to find out some information about Conroe.
[EDIT] Check out www.newegg.com
to help you buying your parts. My personal fav. is Newegg, they have very nice shipping.