For that price, you will not get a "power gaming" system, but an entry-level system. For that price, I would suggest an AMD setup, unfortunately, I don't know much about AMD boards, so have someone double-check what I say :P.
CPU: Athlon 64 X2 5400+ Brisbane 2.8GHz Black Edition
MB: GIGABYTE GA-MA78GM-S2H
GPU: Any 4850 card
(150-160$ after MIR).
With that MB, you even get Hybrid CrossFire with the on-board GPU to give the 4850 card a little boost. Total: 363$, but you are getting 2 free games with that at NewEgg (1 with CPU, 1 with GPU).