That sounds like the default setting... Auto Select (Recommended) which is always good. There's no real reason to dedicate a card to physx. Physx doesn't take that big a toll on a card and not all games make use of physx.
More important is the Manage 3D Settings - Global Settings
in the Global Settings tab. These are the recommended settings according to Koroush Ghazi who used to write game guides for GeForce.com.http://www.tweakguides.com/NVFORCE_6.html