The safer option is another 7850 as Crossfire is proven technology. Theoretically, you could use 650 ti as well but that opens the door to a few problems. I don't have experience running a hybrid system and prefer simpler solutions where possible.
If you really want to use Physx then sell the 7850 for what you can and get a GTX670 which is a neater single card solution at a higher cost.
I find fanboys are a vocal minority on the forums. Most people are pretty balanced. I hope this helps.