By onboard, I mean the video card port on the motherboard it self. Dedicated is normally the word used for a add on card like your 6870.
You should install the latest ATI drivers power down, plug onto the motherboard graphics. boot the system, make sure you have the latest Intel graphics drivers and then the latest version of virtu.
After that, you can test it out and see if virtu is for you or not.
You may also need to set the Init Display First to "onboard"(and not PCI-E or Auto) in the bios. This should be outlined in the motherboard manual(page 49). Onboard VGA should also be set to "Enable Always". This will prevent the board from switching it off when it sees you 6870 card. Both cards need to be running to get virtu to work.
If all this goes well, you will boot up and load windows with the onboard(motherboard/cpu) video card. Once in windows you will want to make sure you have the latest Intel drivers and install Virtu.
If you get to windows and the screen goes blank, swap the screen to the 6870 card. Windows may be there. If that happens, let me know i will try to help you fix that.