Make sure you get your cards with Samsung memory as with a bios mod you can make them go up to 31 Mhash without problems, should do 27-28 Mhash without bios mod though.

I am talking about the best brand in ATI cards,especially for mining, SAPPHIRE NITRO+ is the only kind of card you need for RX 580, just make sure as I said them to have Samsung memory to get the most out of your cards for mining.