NevJF :
I tried playing Batman Arkham City on my GTX 780 and it got a crappy frame rate.
So I checked to see if the GTX 680 could be used as a dedicated Physx card.
If you already have a 680 laying around, then put it in as a PhysX card. For a 780, that's probably one of the best cards to use, but I have seen tests with a GT650 doing very well in a system with a 680 as primary. So, I would say a 660 would be the minimum.