I used a D200 for a long time and it was a great camera. Much of my work involves low light and like you, I ran into some noise problems. However the "chroma noise" looks much worse on the monitor than it did in prints.
Some of the "tricks" that I used to extend my D200 usage were quite elementary but together allowed me to keep on shooting.
o> I spent a lot of time with Brian Peterson's book on exposure. I moved most of my metering to "spot" mode and learned more about "exposing to the right".
o> Noise seems to really pop up in the blue channel. Avoiding fluorescent lighting helped somewhat. I stated using a SB800 with color balancing gels and that also helped. Custom white balance helped in many situations along with the better exposure techniques (above).
o> For a while I used Noise Ninja but found the "noise" sliders in LightRoom to be equally useful and a lot more straight forward.
Roger It's still, ISO, aperture and shutter-speed, right?