I heard that you cannot have a dedicated physx card when you use an AMD card as your primary graphics processor. I plan on buying an R9 290 on cyber monday and I'm not sure if I will just loose physx all together. Assuming that my 850W power supply can support it, I was planning on using the 290 for graphics, and my old(ish) 670 for physx. I would sell my 670 but I don't know anyone who wants to buy it off of me and I don't truse Ebay/Craigslist (otherwise I'd sell it and use something like a 650 for physx).
you used to be able to do it easily, but NVidia squashed it. There was driver hacks even now those don't seem to be working. Sell the card while it's worth something and get something else for your pc. There is only a few games anyways for GPU based PhysX, so why bother keeping a card taht could get you a few hundred bucks.