like i said, my G620 ran for 2 days with onboard gfx (think thats HD2000 though) never tried it with my 2500k, although i have plugged my hdmi cable into the onboard hdmi to see if it saved power whilst watching a film, and it successfully did that. but again im not 100% sure whether that still partially used the gfx cards as they were still fitted.
You need to try it with sandy bridge.