I'm in the process of setting up my first (6-card, I think) mining rig, and I'm having a hard time understanding especially how to choose the GPU.

With comparisons where everything seems to be the same: https://www.asus.com/Product-Compare/?products=nrWaZwKol5KpK4Ud,rxhpVSuR74480kni,1zMvS5oNHEGDTh6Q,CMs4PT6WuXXtfMP0&b=0 how would you know which of these four to choose?

Here are two with a $100 price difference. Are these good prices, and why choose the most expensive:

$750: https://www.msi.com/Graphics-card/GeForce-GTX-1080-GAMING-X-8G.html#hero-overview

$850: https://www.msi.com/Graphics-card/GeForce-GTX-1080-SEA-HAWK-X.html#hero-overview

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For PCU I assume two of these will do: https://www.evga.com/Products/Product.aspx?pn=220-G2-0750-XR

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For CPU I'm thinking of buying a high-end one if there are coins that can be mined with CPU. Please advise.

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8 GB RAM is enough for GPU mining, right? Should this be higher if I decide to CPU mine?

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For MOBO I see the Z270 mentioned a lot. My local shop sells 15, ranging from $150 to $350. Anything I need to know before choosing one?

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Will any old HDD do, or should I choose, say, a 128 GB SSD? I'm planning on using Ubuntu.

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And unless I'm missing something, I need these to complete the rig: http://www.ebay.com/itm/6X-USB-3-0-PCI-E-Express-1xTo16x-GPU-Extender-Riser-Card-Adapter-Power-BTC-Cable-/263070689562

SteinGalen