Go for the 640mb 8800 card. There's not much yet that needs the extra 320mb but when it is called into use it apparently makes a big difference.
Right, but that won't be for a while - even today the difference between the 320 and the 640 versions only starts to split running at 1920x1200, and even then not by much. I run 1920x1200 native (actually 1920x1200+1200x1290) and chose the 320 mb version because it was just so cheap that, in a years time, I could buy the next equivilant card (9800 GTS or something - haha) and not really worry about it. No way would I upgrade from a $1000 (nzd) card in a year!
Although you are dead right about the memory making all the difference, once the card starts swapping it slows down to pathetic speeds. But that's quite an extreme case - I can still run most games at 1920 with 4xAA, and everything at 1920.