Actually, most physX capible graphic engines use physX on the CPU. It's only a few that have more advanced physX effects that requires you to have an Nvidia card to run those calculations.
Even those without Nvidia cards will have physx drivers installed when playing one of the many physx titles. You won't usually have to do this as the game installation will do it for you.
On those games with GPU accelerated physX, you will need an Nvidia card installed, but it can be a dedicated card running along with your other card. If your primary card is an ATI there are hacks that are rather easy to install, that will allow you to do this.