We tested a 8800GTS, was fine on a Q6600 and 4Gigs of Ram, having 40fps (due to some adjustments in Dx and Settings), but overall quality was good on 1240x! I also called someone who has a 9800GTX. I didn´t have much time to talk to him, since he wanted to play the game
EDIT: Thaty why i also stated, that it needs a good CPU too. Having a NVIDIA on a low-end rig, doesn´t mean it still runs better than an ATI on a high rig. But on same rigs, NVIDIA wins the prize.
Evo jos malo o ATI vs NVIDIA