As you'd expect for its class, the D300 has an excellent noise profile up to about ISO 1,600. At that point, softness starts to become evident. ISO 3,200 shots aren't noisy, but they start to lose detail when shot using the D300's default noise-suppression settings, and at ISO 6,400 you're left with a more impressionist than photographic look.
Here you can see the noise/sharpness tradeoff of the various in-camera noise-reduction settings. I tend to think the default choice--Normal--errs a little too much on the side of softness. The grain visible when set to Low is fairly unobtrusive, but the image is noticeably sharper. (ISO 3,200, f/5.6, 1/80 sec, automatic white balance)
It may not be too evident if you're monitor isn't calibrated, but the D300 preserves shadow detail quite well without blowing out the highlights. When zoomed in, you can still see wall details under those lightbulb hot spots. (ISO 1,000, f4.8, 1/30 sec, spot meter, auto white balance)
Here's a simulation of what you can capture with the D300's high-speed burst mode. If memory serves, this was shot with 21-point AF tracking. (As it's in GIF format, this sequence in no way represents the image quality, and it runs at about six frames per second, slightly faster than the D300's tested 5.8fps.) To start the loop again, hit Shift-Refresh in your browser.
In its default settings--both Standard and Neutral--the D300's photos are a hair too soft, even for a pro-level camera. Once I bumped the sharpness up a notch, I was quite happy with the renderings of direct-to-JPEG shots.
It looks like Nikon's gotten a little more aggressive with its noise suppression--some of that comes from the noise reduction built into the sensor--as you can see from this comparison between the D300 and the D200. The tradeoff, as per usual, is a softer image.
Although likely unrelated, note the significant difference in color rendering of the green pastels; the D200's is actually more accurate. (Automatic white balance under very warm tungsten lights.)