actually no, what has been happening more often then not is with a high end card, using a low end GPU for physX actually bottlenecks the more powerful card.
A 580 can do graphics and phsyx faster then a 540 can do the physX.
I Would like to keep the 460 inside the computer for folding anyways. If I get the 660 ti, I should start testing it out, see how much of a frame boost I can get.