So what will give me more fps?
1.Sli config of 550 ti and may be 660 ti?
2.Hd 7850 with 550 ti dedicated to physics?
First of all, you are not dedicating GTX 550 Ti to "Physics".... you'll be dedicated it to "nVidia PhysX". "PhysX" is a restricted and proprietary thing. There is a huge difference and you shouldn't really be bothering with it. It's dead now.
99.9% of the games handle physics via DirectCompute (which both AMD and nVidia support) or CPU (which everyone has). They DON'T handle physics via a propritary thing like "PhysX".
"nVidia PhysX" is a gimmick. Only used by 3-4 games. I'll suggest to sell GTX 550 Ti. You need to decide whether you want to play 3-4 games with additional "PhysX" bling-bling or rest every other game faster.
Funny thing, a single HD 7870 uses less power than a single GTX 550 Ti. yes, HD 7870 is more than twice as fast and uses less power than a SINGLE GTX 550 Ti. You won't have to even change your PSU.
Don't bother with HD 7850 + GTX 550 Ti. Just sell the GTX 550 Ti off, and buy HD 7870 or GTX 660 Ti when it comes out. If you'll be using only HD 7870 or GTX 660 Ti, you don't even need to change the PSU and this is the best thing you can do. Everything else is waste of money in my humble opinion.