In response to the part about mining rigs from your PM (so that others can chime in; also I know all of this info is available numerous times over on the board and the web in general, but since you asked):
It all depends on what you want to mine. I have some decent experience with 1060s, 1070s, and AMD 580s using Ethos. Currently, for price and ROI, the 580s were the best investment, they were all well under $300 and are currently outperforming everything. But that's because I am currently mining electroneum, and my 580s AT STOCK do about 770-800sols. They can be tweaked to get over 1000 I've read (though I haven't messed with flashing ATI cards yet). For longer term, more stable coins, I personally think Nvidia is king, and the 1060 (6gb models, the 3gb will be obsolete before long IMO) is the best buy. At about $300/card they outperform my 1070s significantly in Ethereum and other Ethash coins (dollar for dollar, watt for watt; in strict hashing of COURSE the 1070s perform better). For a starter rig, I would suggest either maxing out your rig with 1060s or 580s, or buying as many 1070s as your budget allows and adding more later with returns (keep in mind the 580s and 1070s are also power hogs, the 1060s are extremely reasonable on draw; none of this matters with Electroneum or other cryptonight coins, though, as the power usage is ridiculously small).
For the rest of your build: cheaper is better. NOT quality, price, so the least bells and whistles for the absolute most stable is going to be your best bet. Basically I add the price of an extra card onto the cost and it usually comes close to covering it:
Motherboard is the trick. You want something simple, stable, and with the number of PCIe ports you're going to want. I tend to stick to 6x or 8x boards; they make mining specific boards with as many as 23 (I think? Maybe it was 21?) pcie slots, but personally I like controlling my rigs on a smaller scale, they're easier to diagnose problems, power continuously (both in PSUs and individual lines, since you're going to need a different circuit for every 12-15 cards depending on settings and your circuit amperage), and control. Plus if one goes down, the other two are still running, so less loss of income. YMMV, plenty of people I know love those crazy farming boards...
Get the cheapest processor that fits the board you pick, it does NOT matter at all (unless you plan to do CPU mining as well, but I don't suggest that).
You don't need much ram, so whatever your OS requires as a minimum will work.
Buy as many risers as you will need (1 per card) PLUS 1 extra per 6. They have a high failure rate, and if you buy the extras individually somewhere like Amazon, they are free to return (but my two cents: keep them around just in case, they're cheap).
PSU: this is the other most important bit. Buy an 80+ gold or platinum PSU. You will be glad you did eventually; many will argue the reliability or safety issues, and I don't buy them for a second, but the real difference is going to be power loss, and remember, rigs run 24/7 so you want the most efficient PSU you can get for your money. Don't skimp here (also be sure it has enough GPU lines for your cards, you'll need one per card, and one SATA line per 2 cards). You will want to figure out the max wattage of each card and get a psu that equals that x number of cards + 50-100W for the system (WAYYY overkill if you shopped right, just being safe) +10-20% depending on how conservative you want to be. Add that total wattage up and find an 80+ g or p PSU that is that number or higher.
All in all, if you're patient (or you can find some lingering post cyber monday sales), you should be able to set up a pretty solid 6 card rig for well under $2K (unless you go 1070s, of course). If you just want to dip your toe in, save some cash and use a computer you already have and just get a couple cards, test them out, and see if you want to expand. That's how I started.
As for ROI, right now the 580s, but I honestly think the 1060s are just a better investment, I think they'll ROI in around the same time, maybe a couple weeks more, but they'll last longer...
