Asus 785G motherboard (the ATX version) has a PCIe x 16 slot running 4 lanes, what would happen if I tried to fold using this? i.e., put a gaming GPU (HD4870) in the x16 slot for actual use and a dedicated folding GPU (9400GT) in the x4 slot
4 lanes of a PCIE x16 is sufficient to run GPU folding - it will not be a bottleneck. Having both ATI and nVidia cards in the same machine gives unpredictable results. Best avoided.