If you're using two NVIDIA cards, yes, you can use the second card for PhysX, but this only gives you any kind of performance gain in PhysX-based games, which are in the large minority out there. I would recommend two cards of the same kind - this way at least you get the 80% performance gain of SLI (If your motherboard supports SLI) on top of the fact you can use one card for PhysX.