Ok heres the backround info. Im buying EVGA Superclocked 570s and I Plan on running them in SLI. Do i Need a physX Card or will the SLI take care of thast? From what I understand you can run a 2/3rd card to do physx Only outside of the SLI. Is that true? Also if it is true what card should I use for a PhysX card?
The 570s will handle the load by themselves. You will have great Physx performance with no real impact to your video performance. Also, keep in mind that only a select handful of games actually take advantage of Physx at all.
Of course, if you try with the SLId 570s and you feel you need more Physx performance, you can add another GPU (doesn't have to be a 570) to serve that purpose. I am pretty sure you won't need that 3rd card.