1. Using several screens, yes. Some games such as Crysis 3 and Bioshock Infinite utilize almost 2 GB of VRAM at my 1080p screen using the older GTX 670. More VRAM will help performance with higher resolutions.
2. If demands are high, then more GPUs featuring 4 GB of VRAM will be made. Simple marketing.
3. I would certainly get the Gigabyte, it features better cooling performance and lower noise levels. The Gigabyte Windforce 3x cooler has cooling potential of 450 watts, which is one of the beafiest GPU coolers on the market.