Is it only me that says get the 640Mb version?
I mean, we've seen games such as Doom3 and Quake4 specifically need cards with 512Mb+ for the Ultra settings. And yes, maybes the IQ difference is neglible, but it's a valid point nevertheless. I would not be happy if I bought the 320Mb version.
But that's just me!
Some examples, taken from,
http://www.digital-daily.com/video/stalker_test/
Stalker with Full Dynamic Lighting, no AA/AF.
Granted you'll be gaming at 1280x1024 but games are starting to use more and more memory. With 640Mb your nice and comfortable, and I would have thought with AA and AF the demand will only increase. Add in ever increasing view distances within games then who knows!
Here is xbit labs,
http://www.xbitlabs.com/articles/video/display/gigabyte-foxconn-gf7950gt.html
All be it a review for the 7950GT, it compares 256Mb against 512Mb. Taken from the conclusion at the end,
I'm not trying to knock the 320Mb card or it's owners. For 90% of the games it'll be fine. I'm thinking for down the line a little bit, or if like me, you're contemplating upgrading your TFT then I think the 320Mb card will be a poor choice.
Up to you though, which ever you can afford.