Wouldn't Windows complain about conflicting drivers between Nvidia and AMD? You may have to do a lot of tweaking just to make them coexist peacefully in the same machine.
I have a GTX560ti, and a Sapphire Radeon 7950 on a Windows 7 box. Already had the 560ti, just plugged in the 7950 installed CCC, and voila. No probs at all.