Lu Heng: this is funny how many people are concern about pixels, high ISO etc., making a war between canon, nikon, sony.. name it. as if it is a substitute of actual photography. as if the one and only available light is a candlelight and they "desperately must" print huge billboards from every shot they take.supposed to be that way?boring.
forget the flexibility, get a 200mm f/2.8L (cheap and great) or a 300mm f/2.8L IS and continue shooting with your 5d2. I believe it would be a MUCH better investment.In my view, neither 5D3 nor D800 are worth it.
I believe the vast majority of posters here are not payed for their photography. A professional would think this 5d3 / d800 matter very objective: how much more money will I make using this new camera? Do some math and if it's worthwhile, buy it. If not (MOST LIKELY!!!), just move on until new gear is really needed.