yes you can usually set profiles for 2d and 3d mode. most under clock the 2d state to do just that. i know evga precision x has that option to save the 2 different modes and most likely msi afterburner
but most cards now use very little power at idle accoriding to nvidia around 5w but i doubt that. but i never really a situation where during normal use im using really any of my gpu so it should usuallybe at idle unless gaming
The GPU will downclock itself to conserve power (2D mode), and will be running at very low speeds. AMD's ZeroCore technology, which is sort of buggy in single GPU configurations (but beastly in Crossfire, the second GPU turns completely off, including fans), also helps conserve power. ZeroCore only helps in Single GPU configurations when the monitor is off.