You have to think differently.
Some cards are good for one or two algorithm some for others.
RX 580 are good for Ethash + Cryptonight (ETH,Monero,ETN...)
It will really depend on what you want to mine.
You can check
https://whattomine.com/ to have an overview of different card.
And think about Hash/PWR because electricity have an important cost on mining.
I have one RIG of 6 RX 580 Nitro + 8G
On cryptonight: ETN I have Hash > 5000H/s for PWR < 800W
On Ethash: ETH Hash > 180 mh/s for PWR < 1000W
So ~200W and 200W in one year cost money.
True. If my powertarget is set to 120% does that mean it will be using 150W*1.20? (180w)