As long as you aren't shooting action in low light, doing really deep crops, or printing wall size, then the D200 really doesn't have too many limitations. I still have pictures on my walls that were shot with my old D50. I agree with some of the previous posters. Unless you determine that a specific subject requires a camera or lens that you don't have, why upgrade?
I started in digital with the D50 in 2005, when I shot mostly landscapes and travel photography. In 2010, I upgraded to the D90 because the D50 didn't give me a fast enough frame rate to get enough keeper shots of youth football and baseball. In 2011, I upgraded to the D7000, because the D90 wasn't good enough under indoor gymnasium light to shoot basketball. I may upgrade again to the D7100 this fall, because it's AF system allows faster, more accurate lock-on for sports, especially indoors.
All of that said, if I still shot only landscapes and travel, I'd probably still be using only my D50.