I was thinking of going 2 gtx 670 sli cause i want to wc, i was wondering if i add a 650 or 660 for a physx card will it boot performance with the 670's sli, also if i should i believe it will slightly future-proof it with the extra hp. what do you think?
Don't bother. Even a single 670 has far more than enough horsepower for PhysX to be a negligible load. Also, if you really wanted a PhysX card, something cheaper like a GT 430 would be a better choice.
No. You missed my point. You shouldn't get any PhysX card. It won't make a difference, because the 670s don't need the help.
For "future-proofing" (there's no such thing), you'll be better off putting that $100 toward your next big upgrade, your next processor/mobo or GPUs.