The 512MB Nvidia 8800GT and 512MB Nvidia 8800GTS are both built around the G92 core.

The 320MB Nvidia 8800GTS and 640MB Nvidia 8800GTS are both built around the older G80 core.

In terms of performance, the 512MB Nvidia 8800GT, 320MB Nvidia 8800GTS and 640MB Nvidia 8800GTS are about the same. The 512MB Nvidia 8800GTS is faster.

Also, the 8800GTS is supposed to be quieter because it has a bigger/slower fan.

More details here