Yeah you don't want that PSU running one card let alone two cards. Or even a new more powerfull card. That PSU is on tier 4 and tier 5 is the worst. That PSU if it was a 450w then is would be fine.
You will see when I link you to it. I don't think you can throw up the old GPU. I would do the SLI first. Look at how many people are still using the GTX 260's. So they will still be good for a while.
If you get a new PSU look for the brands Corsair, Antec, Seasonic, and XFX. That would be your best bet. And I do wish you good luck on you problem.http://www.eggxpert.com/forums/thread/323050.aspx