Did you try downloading the latest version of Nvidia PhysX for your ATI card . I too downloaded it for playing Batman:Arkhum Asylum for my HD 5870 card .
Hopefully someday AMD will have proper PhysX possibilities and we stop this annoying different parties using different options. NVIDIA DRIVERS 9.12.0613 NOTE :
PhysX runs on the CPU.It will work but it will work much less effectively than if you had a gpu-based physX solution (like a single Nvidia card or a dedicated nvidia physX card paired with either an ATI or another nvidia card as your dedicated rendering card solution), meaning that you can run in your system an ATI card and bump some physX into the games that have it.