ive ran physx cards along with my main card but have since remove it. Your main card is in the first slot and the physx card is in the second slot for it to operate so two pci e slots. If you have plenty of fast cpu cores the differance is about 10fps or less from running your proc for physx calculations vs the gpu. Rule of thumb is half the shader cores of the main card as a min or it could cause your system to run even worse. My 280gtx with 250gts doing physx playing mafia2 on the high settings and physx set to high was like 14fps with 3d vision enabled but even without 3d it would only average 24fps or very unplayable. I tried with my 560ti and my own oc with my 280gtx as physx and i had an average of 45 fps (same settings) without 3d and that was just do-able.
It can add alot to a game if your looking for it but really, unless someone is pointing everything out, most effects will be overlooked during gaming.
If my 560ti cant handle bf3 its going to become a glorified physx card when Nvidia refreshes their lineup...................if any other games ever decide to use it again and theres nothing coming down the line that i know of this year.