mid range rig?

[devilsadvocate]

Correct, I wouldn't go lower than a 1070 in the GTX 10 series. Outside of that, a used GTX 980 Ti or GTX 980 should be within your price range.

Hey friends,
thanks to you guys, we got four 1070 gtx cards (Founders edition), there was a good sale and we took advantage of it, thanks to the unanimous opinion here.

Our problem is now, we need the rest of the hardware and i have no idea what is a "must", we want a machine that can run the 4 cards on 100% and even have the ability to expand it with 1-2 more cards in the next few months.

These are our questions (Should i open a new thread for this?)
Cooling - shouldn't be much of an issue since this will sit in a cooled server room, correct?
CPU - I head that if i want to run 4xGPU's(or more) the CPU has some requirments, can someone elaborate?
MOBO - i know nothing about PCIx and such, do i need a mobo with 8 PCIx16? is there even such a thing? and are there "special" mobos that allow some space between the cards?
PSU - do i simply go for recommended and multiply by 4? what do people usually take as a PSU for 4-6 cards? (1070s)
case - i assume we should just get a full tower to make sure there is space between the cards?
HDD - im pretty sure there is no need for an SSD, right? if i have tons of dictionaries(which i do) the SSD's will be too small, will that effect performance? and if i have a small SSD 'c' drive, that has the Hashcat folder, will it make it run faster?
Any other hardware related things i should know?

P.S
i failed to check the FAQ, so ill do that now. if some of these questions have answers there please ignore them, ill see it there. sorry!

6 cards in one rig is really not optimal from a power perspective.  An overclocked GTX 1070 FE usually runs at close to 170 Watts.  Assuming that those cards are running at full load means 6 x 170 Watts of power consumption, 1020 Watts.  1020 Watts doesn't include the MB or other components.  Just to be safe, you should probably go with a PSU that can handle at least 1300 Watts and plug that rig into it's own dedicated circuit in your home/office/wherever.  FYI: most 15 AMP circuits max out at close to 1800 Watts.  If you have other appliances plugged in to the same circuit, it could affect power stability and you even risk blowing a fuse on that circuit.

Also, motherboards with 6 PCI slots do exist, but the vast majority max out at 4.  I'll let someone else comment on options with Supermicro and Tyan boards.  There might be some options with those, but they might not be practical when it comes to cost and power consumption.

Don't be too concerned with the spacing between the cards.  Even under full load, if you have the fans running at 100 percent, the temps will usually not exceed 65 C.  With my GTX 1080 FE, I run the fan at 80 percent for SHA1 hashes and it never breaks 56 C.  If you are really worried about cooling, you could go for water cooling, but it really isn't necessary with the latest Pascal cards at factory frequencies.  My advice is to just turn the fans up.  Water cooling is too expensive and too much maintenance.

Regarding the SSD, I would stick to a large capacity platter hard drive and spend your money on extra RAM instead, but that's your decision.

If you have the coin, consider building two rigs instead of 1, each with their own dedicated electrical circuit.