Dedicated PhysX card

GT 640 for about $100. It really depends on our budget. Below that, the number of CUDA cores drops precipitously in the GT 630, 610, etc. Try to stick with the 600 series though for less heat and power consumption.

The best option would be another GTX 650 Ti Boost in SLI.
 

GTX 650 Ti's in SLI will perform around the same as a GTX 770, so that is easily your best option. Of course, the 7870 would effectively shut you out of the PhysX club.