Forcing a video card to render 3D and PhysX in the same time is not the optimal setup. You would need an additional nvidia card and set it up in Nvidia Control panel for the second card to do the PhysX acceleration alone. I heard a 9800GT should do the job but you could also go with something more modern like a GT430. So look for a used nvidia series 8000 or 9000 in the high end and you should be ok. Review here
http://benchmarkreviews.com/index.ph...1&limitstart=8