is it worth it

No. Not at all. If you had a single 670 then I might think it's worth it, but two 670's should be a powerhouse and crush all physx applications/games around. I run a 7970ghz with a GTX260 for physx just because my 7970 doesn't support it, but a setup with two 670's should be just fine.
As others have said, no. PhysX only uses up a tiny fraction of the combined power of those two cards.
jeez, what website are we on here?

you should get another 670 for tri SLI