With that setup, you need to go big in order to avoid slowing down your performance. Really, I wouldn't recommend anything less than a GTX 660 or above with that setup. There's very little in the way of benchmarks on the topic, but here's something to give you an idea of what we're talking about.
http://physxinfo.com/articles/
The conclusions of this article support the idea that a good PhysX card needs both a high number of CUDA cores and a high clock speed. That explains why a GTX 650Ti seems a little better than a GTX 680. Honestly, if money is not object, for competitive purposes, I would be looking at a GTX 770 with a high clock speed and high CUDA core count...