Actually modern motherboards automatically switch the built-in graphics off if there's a video card installed. I've noticed that even installing drivers for the onboard chip may fail in such case, the installer just informs that there's no device for the driver. The only advantage of using the Intel chip alternatively is some minor energy saving, and even that might not be true if the Nvidia card were seated. Definitely not worth the effort. In laptops they have such a feature, but that'd be hard coded in the bios, I guess.