the games you mentioned don't even use physx. you need an 8600gt or better for physx in the 5 or 6 games that actually use hardware physx. realistically a 9600gt or gt240 should be the minimum used for a dedicated physx card.
now on all the other non physx games out there the physics is done on your cpu anyway.
For future reference, physx is a joke and does nothing in almost any game, you get a visual improvement (but no gameplay improvements) in a few games, and nothing at all in most games.