Doesn't matter,there wont be any difference what kinda chipset you have,i run an ATI card on an nvidia Nforce 4 chipset,no problems here and also the Nvidia chipsets are better than the ati ones somewhat.
Nvidia has made some very steller chipsets. The nForce4 was basically THE chipset for Athlon 64/X2's! I don't know as much about their newer ones, but unless you plan to go with a Crossfire configureation, I'd say go with nVidia. I assume the newer 5xx chipsets aren't Crossfire compatable?