We've heard that the Radeon R9 390X will arrive in both a 4GB and 8GB version, but the latest rumor is that AMD will release a Water-Cooled Edition as the Radeon R9 390X WCE.
The news is coming from a leaked slide which shows the R9 390X featuring 4096 shader units, DirectX 12 support, "optimized for 4K gaming & beyond", and it will arrive in a "special enthusiast water-cooled edition" with up to 8GB of ultra-high bandwidth HBM video memory. The more we hear about the R9 390X, the more we want to see it.