Til I got this big ol' hunkin' GTX 260 my comp was only 500$, and it ran AH dang near perfectly.
AeroCool M40 Case - $80
Biostar AM2+ Mobo - $55
AMD Athlon X2 4850E - $65
2GB DDR2-800 RAM - $35
GTX 260 - $180
320GB HDD - $50
OCZ 750Watt PSU - $80
DVD-Drive - $25
Cost - $570. Before the GTX I used a GeForce 7900 (bad VRAM killed it) which cost, at the time, $90, bringing the cost down to $480. Of course there are places that price can vary, Case can be be cheaper in exchange for a better GPU then 7900 for example.