Original image @ ISO 100.
ISO 100 shows plenty of sharpness and fine detail...
...as does ISO 200.
At ISO 400 and noise is still being kept at bay and the image retains plenty of detail.
At ISO 800 the D800's sensor is still doing an admirable job of keeping noise at bay.
By ISO 1600 there's there merest whiff of noise, but you really need to look hard to spot it.
ISO 3200 is the cut-off point, after which noise becomes more prevalent - especially in processed JPEGs.
It's not difficult to spot noise at the maximum setting of ISO 6400, however it's still possible to make a perfectly good image.
The first extended setting of ISO 12,800 ('Hi1' on the dial) shows a marked deterioration from ISO 6400.
The top extended setting of ISO 25,600 ('Hi2') is fairly noisy, although Raw images can be normally saved - at least for use at smaller sizes.
ISO 200 (for comparison)
ISO 12,800 ('Hi 1')
ISO 25,600 ('Hi 2')