Doh ! Read the manual. that came with the board, you need to find the section in the bios menu that says
System agent configuration agent. Top of the sub menu list is where you enable internal or disable.
or tell the board to boot from external graphics card first. ok. Look at section 2.5.2 to do with the bios config in the manual.
You need to set the options first in the bios, with just the on board graphics chip on, set the settings to use Pci-e interface as primary graphics card.
After that save the bios settings. Power the system down, and insert the Pci-e card into the blue slot only. Make sure you connect the extra power feed from the Psu to the Graphics card if required, and also make sure you plugged in the extra 12v power connector located next to or near where your cpu sits on the board, as this also provides power to the Pci-e slot from the main board. If at any point you failed to follow this. in the right order then its why it is not working with a card in the Pci-e slot but it is with just on board graphics chipset.
Pdf manual download link.http://uk.asus.com/Motherboards/Intel_Socket_1155/P8Z68M_PRO/#download