There is no such thing as an "external" video card. I'm assuming your talking about adding one into the expansion slot, such as PCI, PCI-e, or AGP. For newer computers, the BIOS will automatically detect that another video device is plugged in and will disable onboard video appropriatly. For older ones, you'd have to either disable onboard video in the BIOS, or disable it in the Device Manager. Same goes for the sound card. When installing a new video card, make sure you uninstall the drivers for the onboard video. Turn off the computer, and install the video card. When you boot up, Windows will automatically detect the new hardware, and you would put in the disc and install all the necessary drivers. Any questions?