Dont know why you people make it so complicated.
http://www.parallelminer.com/product/hp-1200-watt-power-supply-kit-for-gpu-mining-platinum-94-zec-dash-eth/Power the PCI-E power connectors this way, hell you can even power the risers with it, it doesn't matter from my testing because the hot of one plug doesnt run into the neutral of another plug. Just turn on the PCI-E/riser power before you turn the mobo power on. It doesnt even matter if you switch them on and keep the mobo off, the GPU gets idle power but generally this is never enough power to even require the fans to turn on (passive cooling will keep the chip below 40).
I looked in to using a server PSU, but decided against it after hearing they are too loud to use in a living space. I have enough noise already with all the auxillary fans I have going. The EVGA PSU I bought has an ECO mode that keeps the fan off untilit reaches 40% load. The fan is also barely audible.
The way he explained that the motherboard PSU should power all the risers was that otherwise it creates a situation where the 2 PSU's fight to regulate the 12V line.
https://bitcointalk.org/index.php?topic=1843586.msg18399752#msg18399752