I see your MB has 6 slots PCI-E, but only powered by 750W PSU
With RX 580 8GB to mine (assumed) Ethash, you will draw 135W. Therefore, you can only use 4 cards (135W * 4 = 540W)
Go here to roughly measure wattage and profitability https://whattomine.com/

For cards, most of the time go for the cheaper one and look for Samsung memory.

You should post in: https://bitcointalk.org/index.php?board=160.0