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To disable the onboard graphics, go into your BIOS and look for a screen with all of your onboard devices (onboard LAN, audio, video etc.) and when you see the onboard video simply change its value to Disabled (every mobo's BIOS is very different so its tough for me to say exactly what to do unless you tell me which one you have) of course you would first turn off the PC, install the 7600 and plug your monitor into the 7600 instead of your onboard before you disable the onboard
So you would uninstall any video drivers you have, install 7600, then plug monitor into it, then reboot and disable the onboard and then reinstall the newest nVidia drivers (84.21 at this time).
It is handy to have those drivers downloaded prior to installation so you can simply double click them and install rather than have to hunt them down on the internet with the horrible graphics you will have with no drivers (like 640x480 and 8 bit color lol)