AMD/nVidia hybrid?

My current computer is AMD all the way through. Phenom 9550 CPU, 790FX chipset and a Radeon 4870 video card.

I was considering the possibility of tossing an nVidia card in just use for Cycles rendering.

Has anyone tried this or have any experience doing this? I know in the past people have done something similar just to use the nVidia card as a Physx processor on AMD systems.