Since the release of the 186 nvidia gfx drivers, Nvidia blocked the physx feature if ever there is a non-nvidia GPU present in your system.
But if you google "ATI physx" you should find a patch/fix that allows you two enable physx on you setup. The problem here is that the patch will not be support by Nvidia and there is no guarantee that future gfx drivers will be problematic etc.
Maybe someone in the community that has a ATI/Nvidia card setup using a patch to enable phsyx can comment?
The other question is "is PhysX really worth considering?
Looking at the review on Tom's website : Batman: Arkham Asylum: GPUs, CPUs, And PhysX Performance
Quoting from the conclusion
"There is a high price to pay for PhysX performance, but I have to admit that the eye candy is a lot of fun to watch. Once you've turned it on, it's not something you'll turn off if your hardware can handle it."
The other thing to consider is that the UT3 engine supports PhysX and since there are a few games that make use of the UT3 engine, ergo ...
(you can also search wikipedia to find all the games that have physx features compared to the games that use havok )
Another thing to consider is DX11's feature- GPGPU, which might make GPU processing available to all gfx platforms, i don't know enough about it but it will be another way developers can use physics in their games...
IMO - I would rather stick with DX11 card rather than a physx enabled card. Or wait until nvidia release their geforce 3000 series (rumours say march/april?)