Nowadays things depend a lot on the particular game. The only situation where the 8800GTS overtook the 2900Pro heavily was in Lost Planet running in DX10 land. Every other situation the 2900Pro held its own pretty nicely, usually slightly better. The main caveats with the 2900 is that it sucks about 25W more juice at peak (during games, and according to their particular test) and it's significantly louder (7 db). The AA thing is complicated because AA is not as important at higher resolutions, only at lower resolutions where these cards are overqualified.
I'm looking at Newegg right now and the cheapest 2900Pro is about $280. However, the 1GB model is 'only' $340, so if you think that VRAM overhead will pay off in the long run, that may be a good choice. Otherwise, the 8800GTS is basically the same price (with rebates), so close in performance most of the time that there's no clear winner.