I believe only something like 9 total games support GPU accelerated PhysX - it is a heavily marketed feature from Nvidia and their fans, but it only applies to a couple of games.
Also, you can still enable PhysX features without an Nvidia GPU - it just carries a heavier framerate hit since it will run off the CPU.
In the couple games that use it, it's a cool feature. However, the VAST majority of games don't use it and that's the ongoing trend - most games use things like the Havok physics engine instead. PhysX is 90% a marketing tool.
^^ It also drops down the frame rates a bit If you have cash you should buy a very cheap dedicated physX card if you are crazy about it. I LOVE PHYSX!!!