Quick update on my GTX with Vista 64bit SP1.
On rig in sig, 175.51 drivers - performance is on par with XP x86, on par being + or - 1-3%.
Crysis DX9 (DX10 unplayable at same settings)
HL2 + ep1 + ep2
No pausing or stuttering
No hanging with Dreamscene.
Desktop and all apps very responsive.
grey's a happy camper.