Very few if ANY PSUs have a temp monitor that's accessible to the end user. None that I'm aware of allow it through software. Sorry. You're best bet is a thermal diode.
Not 100% accurate but I go based off the hardware in use and also Kornowski has a wattage sensor on his case running similar specs with a 8800GTS SLI setup using 500w or so.
Based off that I wager 550w is my 100% load so I am well under my "80% load" goal thus you can't be to far from myself.