If price was of no concern, I would go with ATI for better performance in UT2k4, Splinter Cell, Halo and Far Cry with AA/AF enabled. Being able to beat Nvidia in these 4 games alone at high settings makes ATI a faster card in my eyes. Personally it doesnt matter to me if NVidia beats ATI by 1000 frames in less intensive Open GL games like Quake 3, RTCW, and Call of Duty (esp. when ati already gets 100FPS in those games at highest detail settings so who cares if you get 100 or 200 frames). On the other hand, for the 4 games mentioned above performance matters much more as they are demanding.
As for personal choice, X800Pro and 6800GT seem more appealing, but I wont make the call just yet until the price wars settle down.