I play WoW with an ATI 4850. My game runs better than my gf's who plays with an nVidia card. We bought ours at the same time, I don't remember what card she uses but I know it's the equivalent of my ATI card.
Personally I think this is a silly thing to be worried about. Regardless of what graphics card or processor company logo you see on the box, the game will run worse/better or be more/less optimized depending on dozens of factors, not just what card or processor you use. Your entire PC setup will affect your game performance, right down to your RAM and HD speed, even to what ridiculously flamboyant processes you have running in the background.
This is marketing bullshit.
The majority of software companies optimize their games to run well enough on both. Sometimes ATI will turn out better, sometimes nVidia will turn out better. It depends on the game and your complete hardware setup.
I have been an ATI/amd fan for years, but I will be switching over to nVidia and Intel soon; I find their processor/gpu run much cooler than ati/amd. To me that's more important than raw processing or graphic horsepower - since these days the differences are negligible when you're not playing FPS games that require 400 frames per second.