I would suggest, and i'm sure everyone else on here, that you build your own computer useing these two site.
You will save money by buying all the parts seperately, and putting them together yourself. Plus, you will gain a lot of knowlage about how a computer works. Also, I'm sure a lot of people here would say stick with an AMD 64 3700+ or even an Opteron and not get a dual core proc. You could use the money you saved to upgrade your video card, or even a nice 10k hard drive. What kind of budget do you have?
What I would suggest....
DFI Lanparty SLI-DR - ~$190 at newegg
2x eVGA 7800GT
AMD Athlon 64 3700+
Opteron 146 (I've heard these can OC up to 3GHz.)
For the memory, hard drive, and PSU, someone else will have to help you out with that.