I went through a Vista64 upgrade recently on my monster system, and it made no difference whatsoever to frame rates. It did, however, give me more memory to play with (2GB more since I have 2GB video memory!).

"Gee, so what... that's not very helpful, is it?!". Well, no, but it does mean there's nothing inherently wrong with Vista64, for whatever that's worth

The one oddity for me was the drivers. Vista64 completely failed to recognise my pair of 3870 X2 graphics cards. Completely.

Instead, it installed a generic driver and THAT obviously gave me unplayable frame rates.

So... and please don't take offence at me perhaps Stating the Bloody Obvious(tm)... have you downloaded and installed the latest 64bit drivers for your 8800 GTS? Have you tried an older driver? Checked Google for any anecdotal evidence of problems with various drivers?

My money is firmly on the video driver being the problem.