You can't sli a 560 ti with a 660 gtx, different gpus, only with another 560 ti either 1GB or 2GB, but not the 448 cuda cores one(it's a different gpu).
You can put the 560 ti as a physX card with the 660 gtx.
Put it as a Physx Card?
I wouldn't know how to do that =/
So would be be worth buying to GTX 660 Zotac as a replacement card?