Thanks guys definitely an idea. I have a 295x2 but yes I want 8gb. I also want about 50% more power than the 295x2 to bring some demanding games to 60fps.
I hope Matt posts his findings
edit: looks like i would be doing 290x soundcard 290x 290x no slots free.
Buy a second 295X2 if you don't want to wait for the 390/390X.
DX12 / Vulkan / Mantle can all unify memory for multi-GPU cards btw, so 2x4GB on the 295X2 is not an issue if you choose to go 2x4K in future, for example.