Currently when you add AA the 2900 falls while the 8800 keeps going
There are the same number of nVidia sponsored (where the game is "optimised" for nVidia hardawre) as ATi sponsored games.
The 2900 Pro is an overclocking master (ie it will easily flash to become the £250 XT model).
The drivers will get a bit better, and thus the card will be a little faster, but i'm not expecting an amazing extra amount of performance to be wrung out.
ATi do have a lot better AA quality though - ie on an ATi 4x AA looks about the same as 16x AA on an nVidia card though (from what i've heard), so the big perfomance lossess when you add AA isn't such an issue.
IMO the 2900 Pro is to get back some cash and get a bigger market share for whatever comes next - the 8800's were dominating the DX10-high end, and the long-in-development 2900 XT didn't have good sales.