I'd vote 640
GRAW2 is out in the next couple of weeks and that uses more than 320mb for it's "High" setting on textures, you can only assume that all games after this will most likely be the same. Basically the 320 will fly if you set it to Medium, but slow to a crawl just by enabling High res textures. Where as the 640 will still fly set to High res textures @ 1280x1024
e.g. GRAW, Doom3, Quake4 etc. all use more than 256mb for their High textures, around 280-290mb I believe - which means they play fine on the 320mb, but I know from bitter experience that a 256mb card has to be turned down to Medium textures to play them)
I game at 1280x1024 and I have the 640 for this very reason, it was a lesson learnt the hard way when everyone on here told me to save the money and buy a 256mb card when I was looking at 7xxx series nvidia cards
the 320mb cards came out a bit later, so tend to come factory overclocked, but my 640 overclocks to 620/1900 (core/mem) and I then get scores and framerates close to that of the GTX @ stock