I've looked at other peoples Everest reports and they report:
Real Clock 792 MHz (DDR) (original: 792 MHz)
for the 8800 GTS
Real Clock 396 MHz (DDR) (original: 792 MHz)
[EDIT] I just got a reply from the excellent people at lavalys that makse Everest.
It was a bug in the beta version of the software I used that made my card show up with incorrect speed. It has been fixed, and the new version shows the correct results, Real Clock 792 MHz (DDR) (original: 792 MHz). So while it did look like a problem it wasn't.
Hopefully my new C2D and 680i will give the card a chance to show what it can do tomorrow...