I am going to give you some advice, being someone who has dedicated cards for physx in the past and recently again. A 8600gts isn't going to be enough in newer titles but give it a shot however you will likely have a performance loss. Nvidia has for some time now been raising the bar higher on the demands that physx makes on a system. Used to that a ageia ppu was more than enough and even a meager 8400gs would have managed but now days even your 8600gts isn't going to cut it except for ati users. For physx I wouldn't use any thing less than a 9600gt while 8800/9800gt is often one of the best choices.
Load up the most demanding physx title that you play and run it as normal on the main card. Then perform the same tests with one of your old cards as a ppu. Installation is very easy. I personally believe though that nvidia may have put a cap on how much on a percentile bases how much physx can use.