If you want to run PhysX, then get a low end second card just for PhysX -- not SLI tough.
PhysX does not need a high end card, so get a second hand 8800 GT at eBay, wherever, then set it just for PhysX.
That's a common misconception... You DO need a medium end card for PhysX otherwise it's going to negatively impact your performance... a 8800 gt isn't enough but a 650ti would be enough. (if you go high end, might as well as just set it to SLI instead of dedicated PhysX since it won't fully utilize the gpu)http://www.youtube.com/watch?v=cbww3dhzK0M
The current way that PhysX is set-up, when you assign a GPU to PhysX, that card is the ONLY card that will do PhysX work. If the card is too slow, then it would slow down the more powerful GPU doing the rendering...