While this may well be true, if I still get the performance hit of running 16xAA then it is quite possible to play UT3 on an 8800GT with 16xAA as I have done so (even if it hasn't made a visible difference).
All at the same scene -
No AA, solid 62fps (Vsync)
4xAA, 54fps
16xAA, 43fps
So ner
Did you actually read the reviews? It beats the 640mb in everything as far as I can see (apart from Ep 2 at low res, beats it at higher resolutions), with or without AA at high and low resolutions.
So, you own a GTS 640 or the GTX?