If I get a 780 and use my current 650 Ti BOOST as a physx card would it bottle-neck the 780? I have a 750 watt 80 plus bronze so I am covered in terms of power.
As a general rule, dedicated PhysX cards are completely useless. A 780 on its own will handle any game + PhysX combo out there right now.
You really don't need the 650ti for PhysX. I would sell your old card.
The 780 is a really good performing card. I'm able to run PhysX (the few games that use it) at 1440p with very playable FPS.
Why not test it and then decide for yourself? I don't see how the 650 Ti would bottleneck anything. It's processing a completely separate task from the 780. If anything, I think it would help improve your FPS.