Its smart to have identical power supplies so they turn on at the same time or problems arise.
This simply isn't true. You power the cards first or at the same time. No problems arise.
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#msg18399752Server PSU's can get loud at high loads, but they're pretty quiet at ~70% or lower usage.
If you hooked up a card's riser and PCI-E power to a second PSU, there's no power connection between that card and the other PSU. The card will draw what it needs, there's no fighting to regulate anything, this is just silly.
Source: I run 8 GPU rigs with dual power supplies.
8 cards is a major speedbump in multiple GPU systems. Usually requires driver/bios mods.
They are giving good info. I suggest you read others' threads on multiple GPU systems. Theres LOTS of good info and things to learn just by going through what they did first.
Also,
They also make connectors that link the motherboard power cables so they power on at the same time. Its smart to have identical power supplies so they turn on at the same time or problems arise.
8 cards is zero issue if you have a motherboard that can handle it. You do NOT need to do some sort of BIOS or Driver mod. The only Driver mod required currently is you have to use Catalyst 16.x for RX 500 to run >5GPU, which just involves copying the drivers into a 17.4.3 installer. If you have a motherboard that supports it, it's just plug em in and turn it on.
I think it may depend on the type of risers used. I know some USB risers have a voltage regulator and some do not. I use USB 6-pin risers and checked the outer pins on the USB cable that connects to the 1x PCI-E slot with a multiimeter. There was 3.3V coming from the motherboard. A PCI-E 1x slot also has 3 12V lines connected to it.