I would recommend Gigabyte GA-Z68AP-D3. It costs about $60. I used it to run 4X7990. The CPU is Intel Celeron G1610 (LGA1155) with 4GB memory. The memory speed does not make difference, so you can use cheap memory. The power requirement for 4X7990 is about 1680W (4X400W+80W). I use one 750W+1250W. If you undervolt and underclock, 400W is more than enough for one 7990. The setting, Core/memory = 950/1000MHz, Vcore = 0.99V. The setting varies for each card. You need Afterburner Beta to tune the 7990. Normal Afterburner does not work. The default 1.2V Vcore is too high, it needs more than 550W for each card.

The second choice is MSI Z77A GD55, because it has on board power and reset button. But it is more expensive, maybe about $120. Same G1610. Each 7970 requires 250W if you undervolt.