You have 552W according to the chart I saw on newegg. You basically would be powering 3 130W devices if you did this plus a bit more for your board, ram, drives, etc. So 390 + 50 or around 450W. Your "700W" PSU would be fine.
I say WTF because the 550TI isn't a very good card. A single 470/570 would probably be as fast if not faster. I'd sell the one you have a get a better card.
The reviews I have read of all of the GTX6xx cards (Kepler), have convinced me that every single one, with the exception of the GTX650 (which is weak sauce), is in some way crippled; it might be reduced memory bandwidth, and is almost always compute performance. The latter doesn't matter in today's games (but might in tomorrow's), but it does matter for other tasks. Even the memory bandwidth isn't a big deal if you are fine with 2XAA. The simple fact that they are "crippled", however, has, for the time being (I am not a fanboy), caused me to limit my recommendations to AMD cards (or some of the older Fermi cards). In this case, I think you'd be very pleased with the performance of a HD7870, or even a HD7850.
I'm currently on 2x550Ti's ... I'd go with a 660 or 7850 or something faster. I'm actually in the process of finding my next video card and this set of cards aren't even two years old. Games like Rage and NFS Most Wanted can really choke them out at max settings. I can honestly say however; they don't show much of the microstutter and as a SLi setup the only issues I've ever had is games that don't scale well and EVERY SINGLE UPDATE it disables the SLi and you need to reboot for it to allow you to toss them back on SLi.
I vote for the fastest single GPU you can snag. That's what I'm about to do! LOL