CM Boss GT 550w? I don't suppose you mean GX 550W. If you do mean that - the set-up should be just fine as the 600 series doesn't have that high power consumption. It can also depend on your other specifications and also if you have overclocked anything?
Well in the case, you're going to need a new Power Supply - I recommend getting a trusty brand like Corsair, you could go for something in the Builder Series. I would also recommend going 550W+ to leave any headroom for upgrades.
If you're looking to go SLI I would really recommend a 750w - leaving as much headroom as possible after checking out how much power your system eats is the best way to do it. It means you won't have to face any power issues in the future. Also check that the PSU has the appropriate connections to go SLI!