Hello flaxx
I believe you'll need to find a way to disable your integrated (onboard) video in the BIOS. I know there isn't a direct way to do it but, please, see the next link. The user there found a way.
HERE is the way another user accomplished this task. (disabling onboard video). It concerns a different Asus motherboard but, I checked your motherboard manual, those same features are available.
HERE is the AsusTek forum for your motherboard. You can search and post here. There will be other users of that same board there that I'm sure will be able to help you.
HERE is your motherboard manual. However, I didn't see anything that particularly addressed disabling the onboard video.
Please, post back here w/ your results, questions, and any comments, etc.
Thank you
BJ @:)