This comparison is based upon the Imaging-Resource Still Life standard JPEG ISO test.
I have both cameras and the difference in this comparison between the Nikon D200 and D300 corresponds with my own observations, including using NEFs.

D300 has clearly more resolution and is much sharper probably due to a weaker AA-filter.
The D300 high ISO noise advantage is 1 stop at ISO800 and around 1½ stop at ISO1600 and above.

Lenses used:
D300: Sigma AF 70mm f/2.8G EX DG macro, f8
D200: Nikon AF 105 f/2.8D Micro, f8