Nope, you won't be able to use your current PSU in an SLI set up. You'll want to look at a quality 700+W PSU for SLI.
Okay, minus the SLI, could I add an fx-8120, liquid cooling system, and 2x2gb more RAM?
EDIT: and actually, I just went here: http://www.realhardtechx.com/index_archivos/Page362.htm
and looked at the PSU requirements for a 550ti SLI and it says 500w