Radeon 9550 AGP in Asus P4R800-VM

Hi,

I have just added Radeon 9550 graphics card (256mb) into my Asus P4R800-VM motherboard, running XP.
However, my problem is that I can't seem to set it at 4x or 8x AGP. In the ATI Control Centre it appears to be running as a PCI card. When I set it to 4x or 8x, I get a message popping up saying the system has t orestart in order to test the AGP setting. When I restart and go back in to Control Centre, the settings are back as they were, running as PCI with AGP disabled!
I searched for settings in BIOS, but the only thing I could find (American Megatrends BIOS) was setting the priority in the Northbridge settings from PCI/AGP to AGP/PCI. I even flashed the BIOS but no addidtional setting were added since 2003.
Is there anything else I need to do to make sure the onboard graphics 9100 is completely disabled?
I also tried to find out about the voltage requirements for Radeon 9550 (1.5 volts) but as far as I can tell my motherboard does provide it?
I've also got a 600 Watt power supply, so very much doubt there is a lack of available power.
When running the 3DMark graphics test, my card's performance look bad compared to similar setups (CPU - 2.6BG Intel Celeron)
Any advise would be greatly appreciated!

Thanks :banghead