I have this setup on all my rigs...ez:
1) My understanding is that you want ATX to power risers, CPU, mobo, not GPUs. So, you DON'T need a high wattage here, I uses 750 P2 from eVGA. Without any PCIe splitters, you can power 6x PCIe risers, 1 molex, 3 sata connectors for a total of 10 GPUs. If you want to use Splitters:
http://www.parallelminer.com/product/18awg-pci-e-6pin-to-dual-8-pin-y-splitter-extension-cable-2x-62pin-cable-6in-each-side/ and power more if you need (dont go crazy tho)
2) Server PSUs are great for the mobos. ParallelMiner has several kits avail
http://www.parallelminer.com/product-category/power-supply-kit/ The breakout board makes it really easy to power up the GPUs.
3) On some of my rigs, i have a few risers powered by the server PSUs, and have never had any issues. So, adhere to item 1 as a guideline i suppose.
Powering up: Power on secondary PSU first (server), then ATX...mine away!
Powering down: reverse...Power off secondary PSU, then ATX