There are different ways to eliminate (or at least to reduce) noise in your pictures.
The first option is to avoid noise by shooting with a low ISO value (ISO100-400 on a D200, ISO200 on a D70). The second option is reduce the noise in camera by switching on NR. This is possible for instance in the configuration menus of the D200 to reduce noise automatically when High ISO is selected or when a long shutter time is selected. This significantly improves the quality of your jpeg files. It does not influence your raw files of course.
The next option is to use specific
software, the options being
here numerous. Nikon Capture is the first logical choice for Nikon
users. This
will work reasonably well on raw files. Nikon Capture NX is the most
recent
version (my experience with that new package is very limited, but it
seems that it is an excellent tool in general, but that the noise
reduction performance is a bit disappointing in terms of resources and
throughput time).
Other than that there are a lot of packages or plug ins for classical
packages
like Photoshop allowing you to get excellent results.
One of the recent packages I discovered thanks to a colleague photographer on www.fredmiranda.com, my friend Jacko, is Noiseware. A free version can be downloaded from their site, and it is working pretty well to my standards. Highly recommended.
You can download a version of that package here.