I think no one has a clue how much VRAM games will be able to use once they start taking use of the 8GB memory pool on new consoles and are then ported to the PC.
Give the developers all that RAM, and they'll use it- take Dragon Age II for example with the HQ textures installed. Not a game that requires a lot of rendering power unless you go stupid with the AA settings, but it can use a lot of VRAM, and that game is old.
Those consoles are rendering to 1080p with 7850's- don't expect them to push the geometry or pixel shaders through the roof. Expect them to push the textures to quickly eat up that slack VRAM.
If you're not running 6GB cards now, you're going to want to upgrade down the road just to turn the details up, especially when we start migrating to 4k displays on the desktop (which will likely happen faster than any of us think).
Are you aware of DirectX 11.2's tiled resources, which allows both VRAM and system RAM to store texture data?