PhysX is our physics engine. It runs on any hardware. PhysX is owned by nVidia, but it is a software library running on Intel and AMD processors alike. PhysX GPU Particles require nVidia cards, and newer drivers. PhysX GPU Particles allows us to do some nifty particle effects on the GPU that would be impractical to do on the CPU. In some cases, we forgo doing any of a particular particle effect on the CPU and instead do it all on the GPU if you have GPU Particles turned on, which CAN cause a slight speed increase in those cases. Mostly, however, PhysX GPU Particles is about eye candy, not speed. Hope that clears things up!