Get the new PSU, Your going to need it anyway eventually. Buy the 2nd GTX480 on craigslist/Ebay. Your total upgrade shouldn't be more than $450 which will perform very well.
If the Vram isnt such a issue then...why is nvidia coming out with a 3GB version of the 580?? and even with the 3GB version you still need TWO 580s to due tri monitor gaming...