Lols nice job dude. i've seen a few benchmarks with an extra Nvidia card used for PhysX and they've all been pretty much the same.
From my research, the reason is because the newer cards and process PhysX fast than older ones, and it makes no difference to the rest of the processing.
Heres and easy way to look at it:
Lets say the GTX 480 is split in two and the older Nvidia Card is also split into two. One side for PhysX, the other for everything else. If we were to use an older card, it would use the PhysX side to process the PhysX, but leaves the other side doing nothing. And the same goes for the GTX 480. The "non-PhysX" side will be processing everything else, but the PhysX side is left doing nothing. Obviously, the PhysX side of the GTX 480 is better than the PhysX side of the older Nvidia Card.