Nope. Not that I am aware of. Switchable graphics technology so far only exists for laptops. Generally speaking, modern day graphic cards are pretty power efficient when idling. Most mid range graphic cards can draw 10w or less when idling (i.e. not even watching a video), most high performance cards can draw less than 20w when idling. There are a couple of cards that can be considered "power hungry" when idling; meaning more than 30w. That's the nVidia GTX 580, Radeon HD 6990 and by far the GTX 590 (measured at 49w).
Click below link for the various power states of many (but not all) recent generation graph cores.