I now have the NVIDIA 8800GT in my #2 slot, and my ATI 2600HD in my #1 slot. Two screens are plugged into the NVIDIA, one into the ATI. My question is which does OS X use for main rendering? Or does each just work on the screen(s) plugged in? In Aperture will it use both GPU's to export files, or just one, and if just one which will it choose? Do I need to swap the positions of them?