There should be an option in your bios to disable the onboard graphics chip, or atleast set the new one as the primary. If that doesn't work, plug your monitor into into the onboard, log into windows.
Right click on the desktop, select properties, and click the settings tabs. If there's an option for multiple monitors, select the greyed out one (this should be the new graphics card) and check the box marked "Extend desktop onto this monitor" that will then enable you to check the box marked "Make this my primary monitor"
Finally select the other monitor, uncheck "Extend desktop onto this monitor". At this point, plug the monitor into your new graphics card, and that should be it.