If you research on the steps specific to your board, to remove or disable (the onboard video) it usually involves, [something like] safemode, remove software specific to the onboard video, disable the onboard video. Not possible on all motherboards, as it has consequences on other devices.
On some HP's for example, the best you can do, is in the end so that it use’s as few of resources as possible and still does not kill some other channels on the board necessary for it to run is to have it finally load as a standard vga.
Then load the drivers for your new card.