I don't want to argue but IF you want to play all the new games at highest settings you'll need the 8800 series. STALKER, ArmA, Oblivion, MOHA, Bioshock, CoH, all run very nice on my system towards the highest of settings including AA/AF. However on ArmA I cannot have full view distance, CoH (DX10) I have to lower AA/AF, etc. So IME for the newest games you need 8800, I want to play them at the highest settings, thats what they are made for. Crysis, without violating the NDA, is testing to say the least (insiders can check it out in the insiders forums). If NVidia put out a '8900GTX' which significantly outperformed the 8800GTX (not the Ultra, which is not a huge difference) I would buy 2 today ;) But with older games or engines (e.g. source) you are OK
Look at some of the 30' screen numbers for the ATi crossfire
here
or a NVidia example
here