[Build Help] Is a 600w Power supply enough to run 2 550ti's in SLI?

I already have one 550 ti, and If I could get another on craigslist for like $50 then why not.

Or should I sell the one I have now and get a better card? I am getting a third monitor for Christmas.

Also, with just a single, would I be able to run 3 monitors? Just have 2 extended and one mirrored?