Careful here. Auto is probably best, but if you want to specify, tell it to use both graphics cards in SLI for PhysX. If you select just one of the cards, you will have graphics on one card and PhysX on the other, totally wasting one of the cards in your SLI setup. Having the dual SLI cards perform both graphics and PhysX is best for the highest framerates. Do not have the CPU handle PhysX.