The rated wattage is what the PSU is supposed to deliver. The efficiency is about how much more it pulls from the wall. 650watt 80% efficiency will pull from the wall 650/0.8=812.5 watts at full power. Capacitor aging will have some effect on how much it will deliver over years of use but many quality units can deliver substantially more than they are advertised at.
Normal 670 in SLI calls for a 750watt PSU but yours being a quality would most likely do it but would be pushing it. http://www.realhardtechx.com/index_archivos/Page362.htm