NO, were not speaking about SLI, we speak about dedicated PHYSX card.
you can offload the physx calculation from the main GPU by adding a second GPU (that can be a different one) and tell your driver to use this second card as the Physx driver.
NOT ALL GAME SUPPORT PHYSXhttp://developer.nvidia.com/object/physx_good_company.html
You get better in game physics, not better perf.
Your futur SLI core 216 is still the better choice !
(need nice PSU-CPU-Memory to prevent bottleneck)
In my opinion, buying the core 216 is the better choice you can make. Nice GPU.