Nvidia disables PhysX when ATI card is present
'Since the release of 186 graphics drivers Nvidia has decided to disable PhysX anytime a non-Nvidia GPU is even present in the same PC. We do feel that Nvidia has shot themselves in the foot here.
In NVIDIA's defense, they have been saying ever since the beginning that they would not support the ATI / NVIDIA combo PhysX wise, it is now a little more definitive though.'
Izvor:guru3d