Need more information about your motherboard brand/model and BIOS.
Why don't you put the card in, but still connect your monitor to the onboard graphics VGA/DVI/HDMI connector from the motherboard? That way your BIOS will see the external HD4650, but will also put out what you say is still coming out through the onboard graphics chip.
If you have onboard graphics, and you're trying to go to an external video card of the HD4650, isn't there an "auto" setting in the BIOS? I would think that should detect the presence of an external video card and use it over the onboard graphics.
Anyway, for the moment I'd leave the monitor connected to the onboard connector if you say connecting it to the HD4650 isn't working. Then you can make your BIOS adjustment, and then re-connect and reboot and you should be there. Yes?