Supporting SLI is just getting certified by NVIDIA and going through their process. It doesn't necessarily mean that your PSU won't support it if it does not have the badge.
If you're going to "upgrade" - you may as well go bigger than your plan. But I say stand pat until the card you want is out, as PSU prices will likely drop and better PSUs will be available.
I know how it is, you started out with more bang/buck parts and now you have a feeling of inferiority...heh. I did it with my very first build (after being out of building PCs for years) in '09 and now I've lost my mind. So, choose your path...wisely.