You possibly can, with modified AMD drivers. Google it, you will find.
Keeping a card just for physx is not a good idea IMO.
Much better idea : sell the 560ti , and the 7870. And get a 7950/670/7970 card. You will be much much happier.
Well my mini hdmi port broke so selling it won't be easy. And I have a 900W psu so I should be good.