My guess is that casual viewing with any modern camera-shot images will be good enough to wow a viewer if the subject can wow. Pixel peeping will definitely show the higher res or when comparing the same shot side by side, minute differences in tone smoothness and detail will be seen. A photo, by itself, with no reference to compare to? No, no difference.
That all assumes low ISO and same size print. What the real difference between great cameras and good cameras is seen only in extremes of conditions....high ISO, wide DR scenes, high speed subjects that benefit from superior AF tracking, and large scale prints. In this, a D800 will allow good images to be captured that in extreme conditions would not even be attempted with a lesser camera. The D200 might be useless in a dim theater whereas the D800 would deliver publishable images. The range of available light portraiture for example would be extended greatly with a D800 over that of the D200. Shooting wildlife longer into dusk or before sunrise is a pretty nifty benefit of a D3s/4 or D800. Wedding jobs just got easier if you have been doing them with a D200, with less concern about optimum light, static scenes, etc because you can shoot with the conditions you find, not what you have to create. Your photography club members will go gaga over it but your Aunt Mable, if shown only 1 photo, shot either by a D200 or D800, will not notice any defect or anything needing attention(of course you photo buddies will not either but give then a computer monitor and 100% crops of both images and they will find one vastly superior) Super high sharpness and tonal range is for impressing photographers and yourself, not regular viewers of isolated images, who only care about the subject. Stan St Petersburg Russia