Take V3 Xeon CPUs, which have 40 PCIE lanes
Has 4 PCIE slots (x16 size) at Gen3 with speeds (x16,x8,x8,x8) or 5 PCIE slots (x16 size) at Gen3 with speeds (x8.x8.x8.x8,x8)
Can "overclock" (full Turbo) the CPU just by clicking a button in the BIOS (OCGenie); make sure you pick the correct RAM speed for the Xeon.
Has an ATX Form Factor size which fits the GPU Rack
Takes DDR4 RAM which is obviously much better than DDR3

To make sure we keep the full x16/x8 speed we are using x16 to x16 (molex powered) PCIE risers so that we get the full x16 data bus from motherboard to GPU.

** Note - The PCIE slots above, at those speeds, are only available for Xeons, due to the 40 PCIE lanes, not i7's etc which only have 16 PCIE lanes. **

** Note - The X99 Deluxe actually has 6 GPU slots, however if you use all them then you would not get the PCIE speeds as given above, which may or many not have relevance dependent on which project you crunch as well as what GPU(s) you use.

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If I get a chance I will take a photo of the GPU Rack once is built in a day or so, and post it as it looks quite good :)

it won't hurt to specify the GPU brand

ASUS X99-Deluxe Motherboard (LGA 2011-V3)
GPUs are a MSI 1080 and a Asus 970.

The 970 is in slot 1, and the 1080 is in slot 2. However the event log shows that the 1080 is GPU 0, and the 970 is GPU 1.
That is why I noted that it is probably best to check the Event Log to see exactly which number each GPU has been allocated.

Anyway, thanks for all the help and information, its much appreciated.

(Remembering of course that GPUs are numbered from zero, not one; and as you stated that each brand: NVidia/ATI/Intel has their own number list)