I ran 3DMark05 on my Q6600/8800GTX and got the following results (looking at the results XML file):
Vista Home Premium 32bit:
CPU - 20072 ----- 3D - 17677
XP Pro SP3:
CPU - 12776 ----- 3D - 18012
Why is the CPU score so much worse with XP...?!
From my understanding, this is due to the fact that Vista is more compatable with Mult-core processors than XP. So that Q6600 is getting used a lot more efficiently in Vista than XP.
Probably: XP = 2 cores used, Vista = 4 cores used?
I haven't tried 3dMark06 since I installed Vista 64bit. I should try that tonight to see how I score since I've got same CPU and similar GPU.