AMD`s RX480 is based on Polaris GPU Architecture with GDDR5 Memory. It exists in 2 editions with 4GB Memory with a bandwidth of 224 GB/s and 8GB Memory with a bandwidth of 256 GB/s. For mining Monero / XMR the difference between 4GB and 8GB Version is small, but due to the larger bandwidth on 8GB Memory, you can expect around 40-50 H/s more on that one.
You can expect a hashrate of about 650-750 H/s when using RX480 cards @ power usage of about 85watts. You can push the card over 750 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.