D300 can produce very acceptable photos even at ISO 3200 when and if the
light is almost daylight as color temp (over 4000 K) with a even smooth
spectrum. For ex the interior of a house illuminated from windows, when the
reflective surfaces are mostly neutral. No matter how low is this light. As
long as the light is of "full spectrum" even very dim, the picture will have
not suffer to much from noise. A second parameter is the dynamic range of
the light in the photo. If the darkest area in the photo is not darker than
5-7 EV, then this is a factor to not develop the noise in an inconvenient
way. So, even luminance and good spectrum are the factors that can produce
pictures with less ugly noise. The remaining noise is more even luminance
noise (less or no ugly chrominance), more like a powder, as film grain, that
sometimes can add to beautiful bokeh, if exists.



From the other hand a bad light source, as the green or orange mercury
street lights with the terrible spectrum, or the dim tungsten lights with
yellow or brown lampshade that produce light of very low temp, often less
than 2000K can push the cameras WB adjustment out of the limits even at ISO
200.



For ex if the light has a temp of 1800K (not rare), the blue channel is
almost absent. In that case if we (or the camera) try to correct the color,
not 100%, but even a little just to look more natural and full, we will
amplify the blue channel even by more than 5 EV,. That means that even if we
shot at 200 ISO, the blue channel will be amplified to even higher than ISO
6400 and then the noise will be terrible.



So under circumstances, we can have almost perfect photos at ISO 3200 and
under other very noisy under ISO 800 (or less). Bad light quality and wide
luminance range of the subject are the enemies of the low noise photo

--
Dimitris M



....I have a friend who claims he shoot a D300 at ISO 1,600 without little
perceptible noise. Yet I have not found that to be the case on my D300.