Ive got almost the same setup as you and it runs like poo. Are you using the new beta drivers?
1280x1024, high, dx10 and I get ~20fps.
Q6600
HD2900XT
4GB Ram
That's a giant HL2 esque water splash for a small pistol round
I'm running the drivers that the Time Shift demo directed me to. Like I said it plays great but during cut scenes the frame rate drops sometimes.
There is no dx10 in the demo afaik, and when the full game arrives, well.. youll just have to go vista if you want it.