How do you think which one will be beater for gaming, I am talking about brand not a particular GPU (some high end from both of theme )
From my personal experience I'd vote for ATI. Their latest cards (HD4870 and HD58070) perform really great regardless of the game you're running.
what about the Eyefinity technology in ATI (does it allow to play on 2 screens?), do you know how it works?
Eyefinity supports 3 monitors on the same card (depending on the model) - mine has 2xDVI and 1xDisplayPort. Plug in monitors and go (I am not sure how much configuration is needed in CCC)
I think that ATI are very fast but software and compatibility are an issue. I think that when I will be getting new graphic card it will be definitely Nvidia, but have to be at leas 200% faster then my current Nvidia GTX280. Just like with CPU will not change it antill the will have something 200% faster then my current Quad q9550. No point to change for anything else.