You can insert ANY AGP graphics card into an AGP slot. The AGP slot is effectively a communications port which allows the main board to talk to the adaptor. The adaptor itself is essentially a computer that deals with graphics, so there is no need for any further compatibility (in the same way that you can connect any two computers together via LAN adaptors - the specifications of the two computers do not have to be compatible in any other way).
Buy the AGP adaptor of your choice, then disable the on board adaptor in the BIOS settings.