FiringSquad has published an article that compares PhysX performance of CPU versus PPU versus GPU. The softwares used for this test are Unreal Tournament 3, Nurien and Warmonger.
Conclusion: for over two years old graphics cards, Ageia PhysX PPU is useful but isn’t able to match the performance of today’s GeForce cards.
Read the complete article here: PhysX Performance Update: GPU vs. PPU vs. CPU
In PhysX FluidMark news, I put a graph that shows CPU/PPU/GPU comparison. The results show a larger difference between PPU and GPU but this is due to the kind of test: fluid simulation.