Making use of an integrated GPU while rendering on a nVidia card?

I happen to have two GTX 670s not running in SLI mode.
However, when I render on them, doing anything else on the computer becomes pretty much impossible because of lags. I heard that one can use the integrated GPU to eliminate that problem.
How exactly does this work? My specs are 2600K + z77 Pro4 + 670s. My display is connected to the first 670 via D-DVI.

Which operating system?

Windows 7 64

I guess you just connect your monitor to the VGA outlet on the mainbord, after you have activated the integrated Intel system in bios. Then you use that as your main display adapter, and just use the 670s for rendering in Blender.

And switch back everytime I run a game ? Cant this be done in software settings?