I'm not convinced it takes significantly more graphics power to drive two monitors. However, I did notice a similar situation on my 2011 imac - the idle power jumped >significantly when i hooked in a second display, and iStat Menus showed it was all gpu power.
My theory is the graphics driver disables some of the power saving measures when more than one monitor is hooked up.
You won't fry your pci slot though - 70deg is fine.