Radeon R9 390 & 390X
It appears the Fiji-based Radeon R9 390 and Radeon R9 390X will be the only truly new GPUs in AMD's 300 series. Based on the new GCN 1.3 architecture, the Fiji GPU offers 4096 shaders and a 1024-bit memory bus with 4GB of High Bandwidth Memory (HBM). Availability is expected around June 2015.
Radeon R9 395X2
The dual-GPU flagship features Bermuda, which basically consists of two Fiji cores with a maximum of 8192 shaders, dual 1024-bit memory bus, and 2x 4GB HBM memory. This card will come last, likely somewhere in Fall/Winter 2015.
Radeon R9 380 & 380X
The bad news is that the Radeon R9 380 and Radeon R9 380X are based on Grenada (GCN 1.1), which is basically a rebrand of the Radeon R9 290/290X (Hawaii) lineup with some improvements to the clockspeeds and perhaps higher power efficiency.
The Grenada GPU has the same specifications as Hawaii, this means 2816 stream processors, 176 TMUs, 64 ROPs, and a 512-bit memory bus with 4GB GDDR5 memory. The Radeon R9 380 series is expected towards June 2015.
Radeon R9 370 & 370X
On top of this, the Radeon R9 370 and Radeon R9 370X will feature the Tonga GPU (GCN 1.2), which made its debut with the Radeon R9 285. These cards feature up to 2048 shaders, 256-bit memory bus and 2/4GB GDDR5 memory. Like its faster brothers, these cards are expected towards June.
Radeon R7 360 & 360X
Then there's also the Radeon R7 360 and 360X which will reportedly feature the Trinidad GPU. There's some uncertainty about the design of this chip as some sources claim this will be a rebrand of Pitcairn with 256-bit memory bus and 2GB GDDR5 memory. Other sources claim it's a completely new design. The R7 300 series may be the first to hit the market, current rumors point to a March 2015 launch date.