It will depend on your Operating System. For Windows 7 theres a hack that supposedly works that lets you use a second geforce card as a PhysX device. Windows XP might support this too. ((I haven't looked that closely at the hack)) Windows Vista can't do this due to its driver display models.
You won't be able to run PhysX on AMD hardware. You'll have to run it off the CPU.
You won't be able to run PhysX on AMD hardware. You'll have to run it off the CPU.