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
gigabyte 5850 is voltage locked (likely also with 5830 and 5870. other manufacturers have some sort of voltage changing, and reference cards are great with 0.0125v steps)