I was trying to find this out as well 5 months ago when I was deciding betwen a MBP and the newly released iMacs.
From what I recall at the time people were saying that the 2600 was better with more modern games i.e. those that made use of its unique features. Whereas the 8600 was better for everything else - I've had no problems with the 8600, it's a great card.
But I think the conclusion was that it really depends on which game you're playing and whether it's optimised for the nVidia cards or ATI ones.
P.S. The 256MB RAM in the ATI card would probably be better for larger size textures than the 128MB one in the nVidia.