As expected, the bigger-format D3 is the best. It's the cleanest, and more importantly, retains the most natural detail. The D3 is also the most light sensitive: I shot at 1/500 and got the same exposure as the D90 and D300 set at 1/400. The D3 is 1/3 stop faster than the D90 and D300 when set to the same high ISO.
The D90 is second best. The D90 has much more noise in the background, and pleasantly, it retains what looks like plenty of sharpness and real, live texture in the subject. The D90 seems a tweak oversharpened in the highlights, while that the darker right side of Mr. Monkey is actually noise, and not monkey fur texture. Look at the random pattern of dark spots in the darker fur, not seen in the D3 example. The D90 is doing a superb job of trying to fool our eyes while carefully alternating between real texture and random noise. I wouldn't mind this, so long as the background noise levels are OK.
The D300 is next to last. D300 images at ISO 3,200 don't look noisy, but they do reduce a lot of the resolution and look more like an image from a point-and-shoot as opposed to a real, living clean image like what you get from the D3 and D90.
The D300 has about the same middle-frequency and chroma noise, but has smudged over all the fine monkey fur texture. The D300 image looks OK, until you compare it with the real texture in the D90 image. Much of what looks like fur in the D300 image is actually colored random noise.
The D300 image has also lost color and vibrancy compared to the D90 and D3.
Looking at exposure, the D300 isn't quite as sensitive as the D90; the D300 is slightly darker.
At ISO 3,200 you're making excuses with the D300, while the D3 still looks great. The D90 is in between these two.
The D200 is the worst. It's as noisy as the other DX cameras, has some hot pixels, and has the least retained detail. It looks terdly, but some of this is because it has less saturation to begin with since it has only Gen 1 image adjustments.
Not only this, but the D200 is cheating when set to ISO 3,200, called "High +1.0." I had to use a 1/250 second exposure to get less than I got at 1/500 on the D3 or 1/400 on the D90 and D300.
The D200, looking crappy enough set to ISO 3,200, is less than half as sensitive to light as the D3 set to 3,200. Looking at these results, I could set the D3 to about ISO 1,400, the D90 to ISO 1,800, or the D300 to ISO 2,000 to be as sensitive to light as the D200 set to ISO 3,200. If you set these other cameras to those slower values, they would look even better compared to the D200 for the same exposure.