560ti is probably overkill for physx in the handful of games that use it. All you need for SLI is another 560ti with the same amount of cores and memory.
If you have a 1GB model, I would not suggest getting another one. The reason being 1GB vram isn't enough for the games you'll need 2x 560ti in (for example, BF3).
If you have a 2GB model, SLI would be a decent option, but the one card newegg has in stock is $245, and that's just not a good deal at all. You could get a 660 or 7870 for less and see some improvement.
So my recommendation is if you have a 2GB 560ti and can find another in the $160-$180 range, go ahead and SLI. If you have the 1GB model, either hold off on upgrading or get this Sapphire 7870