The reason they buy a second (cheaper and weaker) GPU for Physx is to offload the Physx work from the main card. For certain games and settings, the hit from enabling Physx can be huge, and using a weak card to offload that Physx work is a fairly cheap way of fixing it. So if you don't experience major slowdowns due to Physx, you don't need a dedicated Physx GPU.
I just got mine GTX480 yesterday and PhysX seems to be working perfectly fine.
I guess it doesn't answer your question,but I am fine with "just" GTX480
Yeah, your right. And the GTX 480 is a gaming card plus a work station card. And with the power of Fermi and 480 Cuda Cores (low-end supercomputing level) it should not even break a sweat when it comes to the PhysX computations.