The PCI Express* slot will automatically disable the onboard graphics after installing the card; there is no setting at a BIOS level to do so.
Our recommendation is confirming compatibility information with the card manufacturer, if possible test with a different one to narrow the issue.
In the configuration menu of DH67BL bios, there is a video settings line. (I own a DH67GD with same hardware/bios).
Entering this sub menu you will find several choices.
Usually, as said before, everything is in auto mode and your PCIe card should be detected with a monitor plugged in, however there are so much possibilities that it is better to check these settings.
What Power supply do you have , Most HD 6970s Draw about 70 Amps , and have one 6 pin plug and one 8 Pin Plug , with a card like this most Mfg. would say a Min. 550 watt , I would use a 850 watt . Because you have alot of other stuff that eats power. Did you have another card before you installed the HD6970?