SLI'ing a GTX 570 with a GTX 550?

Nope, has to be the same card, example 2 550 or 2 570. Although you could use one for physX, but thats another topic

in short, no. nvidia only allows SLI between 2 similar cards, while if there are 2 mismatched cards in the system it dedicates one for physx processing.

Would I be gaining anything by using the 550 for PhysX? If so, how would I go about doing this? Sorry I'm new.

It just gives better physics in the game. But not all games support Physx. Only a few (like Batman Arkham City, etc.) It might also increase couple FPS as well. I think that's the best choice you can do with your GTX 570 and GTX 550 Ti.

If you play Batman or one of the few games that actually take advantage of that, you might go that route, but you'd be better off selling one card for extra spending money.

Agreed. If you don't play any Physx games, might as well sell your GTX 550 Ti, and use the money on something else, or maybe consider save more money and get another GTX 570 instead for SLI.