Leave cool'n'quiet on and other power saving features on, overclock using voltage offset mode. This guide is for a different processor, but some of the bios settings are applicable to 3870k overclocking. Overclocking the GPU should be as simple as selecting whatever speed you want and error checking via occt. I was able to overclock to 847mhz with no voltage increase, graphics core is not stable above that point no matter what I do.
http://benchmarkreviews.com/index.php?option=com_content&task=view&id=834&Itemid=38
P.S. If you are messing with base clock and gpu frequency seperately the speed setting you choose in bios will be incorrect [on my board anyway] because it doesn't account for the change in base clock. So do the math yourself or use like gpu-z or occt to get a reading on your actual igpu speed.