If you choose the 8800 GTS, make sure you go with the 512 MB version opposed to the 320 MB or 640 MB.

The 512 MB is a newer GPU core, and therefore much faster then the 320 MB/640 MB version. The 8800 GT is almost on par with the 8800 GTX, while the 8800 GTS 512 is almost on par with the 8800 Ultra.

here are a couple of reviews:
http://www.anandtech.com/video/showdoc.aspx?i=3140
http://www.anandtech.com/video/showdoc.aspx?i=3175