Theoretically you can do this. You would have to install both cards and get your system to boot using the Radeon. Then use Nvidia's control software to set the GTS450 as a PhysX card. If you already have the cards I would give it a shot, otherwise I wouldn't really recommend it as the drivers from both companies will probably conflict and cause issues.
There is however a company in Israel that is working on a chipset that would allow you to run Nvidia and AMD cards in tandem. I don't remember the name of the company but they showed off a working chip awhile ago.