5x 280x on a Gigabyte GA-970A-D3P ($90 @ Newegg). 2x Rosewill Capstone 750W PSUs. Undervolted the GPUs to 1.019V, and getting 3.5Mh/s @ 1250W.

Here's a similar rig, but with 290s. https://bitcointalk.org/index.php?topic=7216.msg4380855#msg4380855