Do you have a manual for your motherboard?
It could be that it tells how to change your settings.
I doubt it but it is possible that there is a jumper that you have to switch.
If you don't have a manual you might be able to find a pdf of one on the net.
I am thinking that you are running into a device conflict because AGP only allows one device and you have two when you plug a card in. You need to be able to disable the onboard stuff.