Welcome to Nikonians! As long as you don't feel limited by the D200, use it and be happy. If and when the time comes that you find the capability of the D200 limiting then it will be time to consider an upgrade. For example: if your children compete in gymnastics or other indoor sports where flash is not allowed, and the venues are lit worse than a mid-evil dungeon, you will find the high ISO capability of the D200 extremely limiting even if you have a good selection of "Fast Glass" in your kit. This is an extreme case where you will need both "Fast Glass" (fast lenses with large apertures (small f/#'s) ) typically expensive, and a camera body with very high ISO capability (FX).
I have a decent collection of fast glass as well as a few FX bodies that I use when I need the performance. That said, I continue to use my D200 sometimes side by side and other times instead of the newer bodies when the subject and ambient conditions permit.
Once you master the basics of exposure, it will be reasonably easy to transition from one body to another without having to think about it.