Doesn't help that the gigabyte cards are voltage locked to something far above 1.0875v (Stable at 1015MHz at supposedly 1v? yeah right). Anyways, I have 2 5970 and 4 5870, all reference connected to one mobo and one PSU. 3.02 GHash @ 1v, 965 watts or 3.13 Mhash/joule. Not too bad for GPUs
Which Gigabyte cards? From 7 series only voltage locked cards are newer 7970s. Rest can be flashed with old BIOS