From what I've read the 390/390x will be 4096 stream processors, 4gb HBM-4096bit mem bus and could be competing with the GTX980ti 2560 cuda cores, 8gb gddr5, 384bit bus, 190w tdp.
The 380x should have 4096 stream processors, 4gb HBM-1024bit mem bus, 300w tdp. It is suppose to compete with the gtx980.
The 380/385 probably will be same as 380x with less stream processors with roughly a 300w tdp set to compete with the gtx970.
The 370/370x should be 3gb gddr5, 384bit bus. Competing with the GTX960.
The 360/360x should be 2gb gddr5, 256bit bus. Maybe pairing up the the GTX950.
If your not sure to snag a cheap 290/290x now or wait, I'd wait for he 300 series for the "full" dx12 support. AMD anouced the current GCN based card will support dx12 to an extent but most likely will not support dx12's full feature set. Test have already been done by anandatech with dx12 and Windows 10 beta with a 290x and 780ti with star swarm. The 290x had a 400% performance increase while the 780ti had a 165% performance increase. Neither of these cards fully support dx12 and are still showing big performance gains, so the new cards that do should be even better. Just gotta wait till next year when some dx12 games start showing up.
I also heard on tek syndicate show that dx12 will allow the use of AMD and Nvidia cards in the same machine working together. Also may allow the use of two card that aren't the same.... Say a 380x and 360x in crossfire.... We'll see I guess.