PhysX GPU acceleration only works on Nvidia cards, since it's implemented using CUDA. So, officially is not possible to use PhysX GPU acceleration with a Radeon card.
What will happen if you try to run a PhysX game with a Radeon card? Well, it would work just fine but the extra eye candy from the physics effects from the GPU acceleration wouldn't be enabled, the other regular PhysX effects would be handled by the CPU.
I haven't found a way to enable PhysX acceleration on a Radeon card, but a while ago it was possible to use specially modified drivers to use a Radeon card as a main GPU and add a second Nvidia card as a dedicated PhysX card, but a you mention many of this drivers are old and haven't been updated in a while. Maybe if you...