I currently play at 1680 x 1050. Crysis is the only game I have that I dont use AA for. CnC 3 KW runs fine at 8x AA, so long as i dont have more than 100ish units onscreen. Of which I usually do, so usually 4x AA. And for the crysis CPU bench I got 36.94.
Just depends on the game really. Anyway you're talking about a 30% increase in the number of pixels that your GPU is going to have to deal with which. If your GPU is the bottleneck on a game (especially the GPU fill rate) then that could cost you a 30% drop in performance. If you have the 512MB card then there is more of a concern about you running low on GPU memory and thus causing even slower performance. If you have the 1GB card then I guess that shouldn't be a worry right now .