It's recommended to have at least a 750W for a GTX 260 SLI setup. PC Power & Cooling makes one of the best power supplies available. Since you already have a GTX 260, if you can get another at a good price, it would be worth the upgrade. They scale very nicely in SLI.
Have you looked at EVGA's Step-Up
program to see if you can get a more powerful card such as the GTX 285?
here's a link to most everything you need to know about SLI: SLI Zone