AMD`s RX470 is based on Polaris GPU Architecture with GDDR5 Memory. It exists in 2 editions with 4GB and 8GB Memory both with a bandwidth of 211 GB/s. For mining Monero / XMR the difference between 4GB and 8GB Version is more or less none.
You can expect a hashrate of about 630-700 H/s when using RX470 cards @ power usage of about 80watts. You can push the card over 700 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.