I think the integrated graphics adapter is automatically disabled when there is something plugged in the AGP slot. Or maybe there’s a switch in the BIOS. Check your motherboard’s manual.
Of course before doing this, you should uninstall the Intel driver.
In most motherboards (eg intel 865, 875) when you insert the AGP card, the on-board card will be automatically disabled.
For others (some old ones) you have an option in the bios settings.