I tried that once with an AMD/Nvidia combo setup. If I remember right I upgraded to an AMD HD7970 and ran physx with the old GTX280. Negligible gain at best. If you have a modern CPU then you are already running the physics software. Its part of the nvidia driver package.
I wouldn't waste the time to be honest, or the electricity.
If you want to go dual card get another 970 and run sli.