It's not worth it to buy a 660Ti for Physx. I would get a cheaper card, like a GTS250 or a GTS450 for Physx. The 660Ti is way way overkill. You can even get a 550Ti for Physx, but that's all you really need.
If you do do the Hybrid Physx, I can show you how to do it. Its quite simple.
Since you are contemplating a main card and a physX card, I'd recommend just going with two 670's. You'd get more performance all the time with the SLI, and you also get PhysX. In the extremely rare case you run into a game which needs a card dedicated to run high levels of PhysX (not sure there is a case where this is true, maybe Dark Void), you can dedicate one of the cards for PhysX.
yea but im worried about the visually demanding games like mafia2, crysis, metro 2033 that will lower frame rate
Of those games, only Mafia 2 would benefit from a PhysX card. Metro 2033 uses so little PhysX, you might find better performance just letting the CPU do it (early benchmarks at least, showed this). Crysis doesn't use PhysX at all, most games don't.
A GT640 for under $100 has 384 CUDA cores and should make a great dedicated PhysX card. In any event, I think the strategy is to get a single 670/680, and then add an additional PhysX card or SLI card if you decide through gameplay that you need more FPS.
Borderlands 2 and Metro Last Light look like they are both doubling down on the PhysX effects, so plan accordingly.