Proprietary standards are BS in my opinion. If Nvidia opened, it i might have actually considered BUYING a ppu. Anyways, i did the exact same thing...use thishttp://physxinfo.com/news/942/hybrid-physx-mod-1-02-195-xx-drivers-and-win-xp-support/
Install ati drivers, restart, install nvidia drivers (i think you need 195.xx), then before you restart run this program.
Only works on win 7 or xp (no vista) that supports dual gpu drivers.
I had this setup running for a few months...but then i realized NOTHING REALLY USES PHYSX!http://www.nzone.com/object/nzone_physxgames_home.html
even then, phyx is pretty weak minus Batman...even then do you really need cloth effects?
So i took out the ppu, and now my 5850 runs 20C cooler