I have an NVIDIA Quadro 2000 running under Windows 8 (64bit) without problems. Both PS6 and C1 work fine.
My previous AMD/ATI V4800 had a lot of trouble with Windows 7, mostly driver based, so up- or downgrading the driver would normally solve the problem. In the end I got tired and switched to the NVIDIA card which has so far worked perfectly.
Hope this helps,