I think 4K will be the minimum necessary to have a real VR experience. I mentioned it in another comment: suspension of disbelief is everything, but currently my eyes focus too keenly on the scanlines and my immersion is ruined. It doesn't help that all my consumer devices have retina-like quality, so I'm used to nearly flawless clarity in my displays. When I'm using the 720p Oculus Rift it just bugs me too much. 1080p will be better, but still not great.