Some more answers - I don't need to answer about the HDMI dummy plugs because a couple other people described their use perfectly.
As far as motherboard, I use the ASRock H81 BTC, like most people do I think - most are the older version, but I have two of their newer ones (basically the same). I have a couple random boards when the H81's weren't available, but now that they are again that's my goto board.
On the Memory/CPU, most all have 16GB memory - it's cheap enough, so why not. On the CPU side, I just pick whichever CPU has the best bang for the buck at the time, so I'm not overly concerned about that either. Since I don't mine with the GPU and really just have it run the system, any of them seem to work equally well.
OS-wise I run Windows 10, I don't bother licensing it because unlicensed gives me all the functionality I need, and I went through and locked down the image so it's basically purpose built for mining. Runs off an SSD, usually 120GB just to be safe, but you could easily use smaller.
Mining-wise I use Claymore/WattTool/GPU-Z/PolarisBiosEditor/ATIFlash, and then a couple of random custom things I wrote to make rebooting and doing scheduled tasks easier to deal with.
There were questions about power - and a couple people gave great answers, I'll just throw in my specific solution. In the case of where I have these miners, I have ~300 amps available at 240v, so everything runs off of that. The machines themselves don't even come close to using 2000w anymore - the older ones with R9's did though. Either way, there is some slight loss of efficiency in the PSU to be running basically at 60% of max - but not enough for me to really care.
When you say breaker, I think you're asking about the breakout - and that comes with 6 pin plugs. I buy custom cables from Sidehack, so he makes me 6-pin to 8-pin cables that are great quality and perfect for mining - I highly recommend his cables. As far as whether it's safe to run an 8-pin card off of a 6-pin connector, the answer is yes if you're using high-quality cables, and you know you're not exceeding the connector/AWG max power ratings, which basically none of the GPU's even come close to. Since they assume you're connecting with PSU 6-pin wires, which are generally not that good, you're well within safety when using some miner-grade cables.
For powering the motherboard, I just run a custom 6-pin cable to one of these:
http://amzn.to/2gU8pWPThey work great, and all it really powers is the motherboard and the SSD, which typically pull well under 50w, so it's more than enough.
I dug up an old picture of one of my earlier designs (these fans were worthless compared to what I use now, older risers, I was still doing the 5v stepdown myself, etc) - but it gives a good idea how everything is setup: