Well your adding extra physics to the pot now, so naturally things can slow down a bit. Ageia had a great idea and it needs to be matured. The idea in fact is now being taken into consideration by ATI and Nvidia with thier crossfire and sli cards. So, Ageia must be doing something right. If you are considering buying the card wait a few months and see what happens. As more and more games start to support it the PhysX processor will start to mature as will its drivers and maybe future versions of it as well.
Just my 2 cents.
Video cards can speed up Physics. (or add moving objects)
A bigger problem is getting the physics RIGHT. Things still poke out of other objects, things don't sit correctly, most objects are still immovable, I have yet to see realistic moving water or realistic particle animation, etc etc etc,
Those problems need to get fixed before we should worry about adding more objects.