I'd stick with something in this model.
Asus P5Q-P, or Pro. ~$130
E8800/2200 ~80/140.
E2220's OC to around 3.2GHz, the 8800 to 4.4 with better cooling.
And of course, 8800GT can support most shit you throw at it, virtually everything unless you want crysis AA @ 4x.. which is insane, but you may want a 9900GTX or 260GTX..
anyway, $400 or so in total, and 20x as cheap as a crappy ass 'gaming' computer you could buy with lower specs.
Oh yeah, 600-700W power supply, with atleast 20A on the +12v rails.