I can tell you first hand not to go SLI with the 550's. Even with the second 550 you will not be as fast as a 560 Ti. The GTX 560 Tis some of them are below $200.00 and the non Ti's are for sure below $200.00.
It is too bad you can't spend more money for the new 6xx series cards. The new 660 ti is coming out right around the corner. You have put a good system together from what you posted.
So why skimp on the card that is what I did and I wish I would have not done that. I also skimp on my PSU. If you haven't already got one make sure they are these types of brands.
Corsair, Antec, Seasonic, XFX, Sliver Stone, and some OCZ brands. There are the AMD/ATI cards like the 7xxx series and the 6xxx series of card the 7xxx series being there newest ones.
But if your budget only allows for the 5xx series cards then get the 560 Ti would be the best bang for your budget. I do wish you good luck on your build and so far it is a nice one. Cheers
How big is your PSU. If you're going to SLI you need a solid quality PSU. Also, if you have any reason to feel like you should wait, then by all means wait for the new 660 to come out from Nvidia. Buy it as soon as you can because they will fly out of stock just like their brothers.