I heard ATI cards are better at anti-aliasing and overall picture quality.
Actually , that's because they can enable AA+HDR... and the X1900 series has a slightly better AF .. other than that there is no difference in image quality .
By the way .. ATI cards have 2X , 4X , 6X AA . They can't do 8X AA unless they're crossfired .
Nvidia cards have 2X , 2XQ , 4X , 8X AA ..they can't do 6X AA ..8X is better in image quality of course .. but it costs more performance than 6X .
any Geforce 6 and 7 card can enable 8X AA at any resolution .. BUT the cards limit their capabiltity to do 8X AA to a certain level .
For example : if you have a 6600GT .. you can enable 8X AA @ 1280 x 1024 in Hitman 2 .. BUT you can't enable that for NFS: Most Wanted .. instead you have to lower the resolution to 1024 x 768 to be able to use 8X .. what happened here is that the driver detected that you can't have any decent frames at that level of AA and resolution .
for 7900GT I think it can enable 8X at a resolution up to 1600 x 1200 .. Again that's my guess .. it could be wrong .