A higher ISO is going to produce noise, at some level. That's all there is to it.
While the D300 may be able to produce very clean ISO 800 files, I hardly think leaving it at ISO 800 all the time is a "smart practice". ISO should be changed based upon the situation you find yourself in.
For example, you could have opened your aperture by a stop to f/16 and reduced the ISO to 400. This would have given you the same exposure, but prevented another factor that might be degrading your image - diffraction. Of course, that may have prevented you from getting the star effect from the lights in the background.
Thinking about your settings is a more important than anything Nikon can say about the capabilities of their cameras.
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