Your PSU has 1x6pin and 1x6+2pin connectors. You need 2x6pin connectors for a 460. 4x6pin for 2 460s. I think you can use a molex connector to solve that problem. 460s in SLi are recommended a 600W power supply. Your PSU is more than enough. Your system will not use the maximum amount of power it can use ALL the time, only when it is needed (rarely)... You will be safe.
460s in SLi will be sweet, good luck with your upgrade