Unless you play on >2K resolutions and use HD texture packs in combination with x8 MSAA samples, downsampling and/ or video memory-hungry mods, 4 GB vram should be plenty enough for at least 1-2 years on most AAA games, in my opinion.
The problem with vram though is that it never is enough. You are always bound to hit the wall at some point.
Having that extra video memory in your gpu adapter, especially on high-end gpus, will always render it more future-proof.
For example, i would easily get stable >60-80 fps without dropouts on the most demanding games, and assuming crossfirex is scaling good, on even one of my HD5970s (2 gpus), if only i had 2-4 GB of vram instead of just 1 gb. The rendering power has not become obsolete yet (i have tested this on lots of demanding games), which is astonishing for such an old dx11 adapter, but the video memory has. It chokes performance when > 1gb is needed to be cached. And it is understandable.
With this in mind, i would prefer to get an r9 390x with 8gb vram, or a gtx 980 ti with 6gb which both have plenty of processing power and sufficient memory for current and future games for a long while. An r9 fury x is the least ideal long-term, considering that 4gb would theoretically become obsolete quite sooner, but i would still get one of these as well.