Are you running SP1?

Have you looked at performance counters?

Are you capping your frames when boxing? When you are not?

I don't quite understand your last statement, if you are within 15-10 frames of the way it ran in XP, then I'm not sure what your problem is? Have you duplicated all of your friends game settings on a 1:1 basis, and I mean all of them, on every tab? How is the performance in a maximized window when you launch it manually? Specifically windowed mode maximized.

What about virus/worm/rootkit/malware/spyware?

What about driver revisions? Firmware? BIOS updates?

You do know there is a major difference in the way the memory controllers work between the 8800gt and 8800gtx?

I'll definitely agree with your opinion of DDR2 vs DDR3 RAM itself, the memory controllers are much nicer on DDR3 than DDR2 but are not worth it right now since nothing really can take advantage of the enhancements to that currently in the gaming market.

Edit: Whoops, got pulled away from my desk, some of this may have been covered above.