[KAOOS]

The D3x is usable until ISO1600, but then drops in IQ dramatically. You can use Software to denoise the ISO3200/6400 Files, but the result is far from good, compared to newer Sensors, like D4 or even D3s. ISO100 is very clean and accurate in color, but outperformed by newer cameras. But it is still a very fine workhorse, when using under very good and bright conditions.
Anyway Photozone should update to D810 or D850 soon, as these are the most used ones for FX and represent the majority of users.