Jason P. Odell shared his impressions about D3 and D300 and compared it to the D2X:

“I was playing around last night making some test shots in my basement. I compared a series of images with the D3, D300 and D2Xs taken at different ISOs in horrible light (compact fluorescents). I shot in RAW, NR OFF, and processed using NX 1.3 with identical Picture Control settings. I turned sharpening OFF to minimize any sharpening artifacts in the base images.
I don’t have the crops to show yet, but here are my general conclusions:

1) The D300 does a better job with WB in this kind of lighting (cool fluorescents) than the D2X. Images from both cameras required some WB modification in Capture NX, but the end product looked better from the D300 and D3. The D2X images tended to have a yellowish color cast that required significantly more WB adjustment to remove.

2) With respect to ISO performance, I’d say that the D300 is about 1.5 stops better in terms of noise than the D2Xs. Luminance noise becomes visible with the D300 at ISO 1600, but is not pronounced until ISO 2500. I can get an image from the D300 shot at ISO 2500 that looks about as good or slightly better than the D2Xs shot at ISO 1000 (Hi-0.3). The D3 beats the D2X by about three stops— HI-2 images (3200) on the D2Xs look about the same as HI-2 images (ISO 25,600) on the D3!

My conclusions on D300 noise (qualitative, not quantitative):

With the D300, you can shoot confidently at ISO 2500 or lower and your files will require minimal (if any) noise reduction, depending on the final print size. Because the noise is primarily in the luminance channel, be careful with your NR routine so you don’t destroy image detail. I made a 12×18″ print of the ISO 2500 image from the D300– the grain pattern was noticeable if you looked really closely. However, from normal viewing distances, the image looked great and there was no loss of fine details. ISO 3200 on the D300 introduces significantly more luminance (and more chrominance) noise into the image, but again, images are very easy to clean up and they retain excellent color and contrast (something you can’t say about the D2X).

So if you are thinking about a D300 or a used D2X/s, this round definitely goes to the D300.”