2x7990 or 4x7970

[epixoip]

you will need a lot more than 1000W of power. you should budget at least 1200W RMS to power the GPUs alone, so i would recommend going with a 1500W or 1600W 80plus Gold or Platinum-certified power supply.

by 'reference design' i mean the design developed by AMD, not one with a third-party cooling solution.

these are all reference design cards:

http://www.newegg.com/Product/Product.as...6814103201
http://www.newegg.com/Product/Product.as...6814161399
http://www.newegg.com/Product/Product.as...6814127652
http://www.newegg.com/Product/Product.as...6814129204

anything that doesn't look like that, like the ones that have two fans on the side, should be avoided.