AMD`s RX580 is based on Polaris GPU Architecture with GDDR5 Memory just like it predecessor RX480 – There are only a few slight changes from it predecessor . It exists in 2 editions with 4GB Memory and 8GB Memory of which both have a bandwidth of 256 GB/s. For mining Monero / XMR the difference between 4GB and 8GB Version is almost none.
You can expect a hashrate of about 660-720 H/s when using RX580 cards @ power usage of about 85watts. You can push the card over 720 H/s but that would cost you extra in terms of power usage, as those last hashes usually costs atleast 1 watt each.
Look at our BIOS MOD pages if you would like to max out performance on your card.