Here's a quick overview of all the rumors:
AMD Radeon RX 500 series launching in April.
AMD Radeon RX 580 and Radeon RX 570 to be available for sale on April 4th. These are believed to be rebrands of the Radeon RX 480 and Radeon RX 470 with higher clockspeeds. RX 580 has 74MHz higher Boost than RX 480, while RX 570 has 38MHz higher boost plus 400MHz faster memory;
AMD Radeon RX 560 expected on April 11, believed to be a rebrand of the RX 460 with a 87MHz higher Boost and perhaps 1024 instead of 896 stream processors.
AMD Radeon RX 550 expected on April 11, this may be the only new part. Rumors point to this one being based on the low-end Polaris 12 GPU.
AMD Radeon RX Vega follows in May.