The GeForce 8800 GTX I'm using under Vista 64 has a constant fan at low frequency so it's not obtrusive to me. I measured 50 dB at 12 inches from the back of the Mac Pro.
When I remove it and install the Radeon HD 2600 XT, the level drops to 40 dB.
So the Geforce 8800 GT is outputting more than 3 times the noise the Radeon HD 2600 XT does.
Granted, it is also at least 3 times better performing.
2 different cards aren't they:
GeForce 8800 GTX
Geforce 8800 GT