AMD originally planned on launching their Radeon 500 Series graphics cards on April 4th. Apparently this has been postponed till April 18th, which will be a day after AMD releases their Ryzen 5 processors.
The cards that we know of that will be directly effected by the delay will be the Radeon RX 580 and Radeon RX 570. From what we know these cards will be rebrands of the current Radeon RX 400 series. The cards will be based on the Polaris 10 and Polaris 11 GPUs, we should see a boost in clock speeds though.
There is also talks of a new Polaris 12 GPU which will be even more low power than the Polaris 11 GPU. I guess we will find out more as we get closer to the release date.