Hey if you'd like to buy my 320, I'll buy a 640 for myself. I can't stand the way my 320 sputters and spits in outdoor Oblivion, and that's only at 1280*1024 with all details up and HDR, also with draw distances maxed out.
I think the 320 has real problems with texture swapping, and it is my opinion that the benchmark numbers we see online do not reflect the pauses that the card experiences during swapping.
On the other hand, when the card has all the textures loaded, it processes them incredibly fast, so if you can live with some of the settings turned down, or a little less resolution, you will have a real fast card on your hands.