There are a couple of things that are hard to tell with digital images on a computer screen - how sharp the photo will print and how the noise reproduce in the final product. For both I usually only look at 1/2 to 1/3 size on the screen to try and determine how my prints will look, even for very large prints. In almost all cases the prints come out better than what I see on screen.
For sharpness the D800 has a lot going for it when making images for the web. Because of the reduction in pixel size for computer screens, there is a lot of room for focus or blur and still have decentl image. In my Nikonians Just Some Photos gallery, there is a dandelion that is way out of focus, but when the whole image is viewed it is not half bad, considering. Make sure to look at the large images to see the difference.
When I was first starting photography Byron Nickolson gave me a camera with a 50mm lens and I had to bring a roll of exposed film every other day. When he was satisfied with the photos he gave me a 200mm lens and told me to go shot moving objects. And when I could handhold the camera (50 or 64 ASA film) at 1/100 sec and have acceptable focus I could move on. A lesson that served me well. I still go out and practice once in a while photographing horses on carousels, cars moving down a busy street or on the highway and see how slow of a shutter speed I can get.