Buy RX 560 4G if you are smart.
They can do 15mh/s on ETH are twice cheaper than any 570 or 580.
If you judgment is the price, it's right but if the consideration of profit, it's wrong. If the RX560 4GB is 15 MH/s while the RX580 8 GB defaults to 24 MH/s and Mod 29 MH/s after tuning 30+ MH/s.
Especially if for 1 or 2 year RX580 8GB is good if still mining on ETH. 4 GB will not be used again on ETH when Dag size has reached 4 GB although ETH will in prediction go to POS.