What Gener_AL said
64 bit OSes need more power to run
the most I've been able to get my 9950 to run Vista Ulimate 64bit was @3.250 with 1.55 vcore

WinXP and Vista Home Premium 32bit I can do 3.440@1.55v GHz no problem

Also increasing your PCI-E speed will give you a boost to 3DMark score
I run mine at 119 MHz
Either increase the PCIE Frequency or enable Auto Express and then back it down with AOD
For me, if I did both ,things became unstable.