the 7900gt is a great card.
but if your looking at budget the 7600gt or the 6800xt are both nice cards. i would say either one will get the job done.
and personally i would go with daul core, beacsue it will help you with stuff other than gaming, and in the future gaming as well.
My gaming rig:
ASUS P6T | i7-920 D0 @ 4.0GHZ - 1.25v w/ H50
GTX 460/768MB (750/1500/2000) | 12GB Corsair DDR3-1600 @ (8-8-8-24)
2 x PS-100 32GB SSD Raid 0 | 4 x WD 750GB Raid 5 | Corsair 620HX
Asus 21.6" LCD (1080p) | LG 32" TV (1080p)
Logitech G15 | Logitech MX518 | Logitech MX1100 | Razer Mamba
Heatware | http://www.heatware.com/eval.php?id=59417