I use the setup your asking about, though it's not a milk crate. I use this PC on a daily basis  for everything while it is mining in the background. I got rid of the stock coolers in favor of watercooling. The entire unit is whisper quiet at 100% load and the GPU's never go past 60c overclocked.

Motherboard is a Gigabyte Z87 OC Force
16GB ram
4x R9 290X cards
2x 1000w platinum rated power supplies. Each one hosts two cards and one pump with room to spare.

I don't have any risers yet, but there are two more pcie slots available  incase I need more.

http://farm6.staticflickr.com/5510/12354377625_7a6974f5f0_b.jpg