There may not be a clearly marked "Disable Onboard Video" listing in the BIOS.  If there is not, all you need to do is physically install the video card into it's compatible slot, boot up the system, during the initial post screen press F2 to enter BIOS.

Once in BIOS goto Advanced Chipset Features, then select Init Display First and select your bus type by using the arrow keys to highlight the correct option the press enter.  Press F10 to save and exit.  Plug your monitor into the output of the video card and you are ready to star the new system build.