8800GTS Question

Hi,
I've decided to get an 8800GTS for my pc, but what version is best to get -320Mb or 640 Mb. The 640Mb card seems to be going for about
I've decided to get an 8800GTS for my pc, but what version is best to get -320Mb or 640 Mb. The 640Mb card seems to be going for about