There is a driver crack that allows you to use an Nvidia card as PhysX with an ATI primary card. Nvidia drivers disallow this by default which is why you need the driver crack. The only thing is that the Nvidia card sits there doing NOTHING unless the game actually uses PhysX, not to be mistaken with Physics. There are only a few games that actually incorporate this, Batman: Arkham Asylum being one of the noteworthy. I've actually seen a few reviews where having the card sit idle actually causes you to loose 1-2 FPS in some games.
I had a link to this, but no longer do, I'm sure you could search it if you want to. I was actually going to try this with an 8800GTX for physX but honestly you end up with a card that just sits there sucking power unless you are playing a game that uses this. I don't play any games that use PhysX so it was pointless.