Currently I am happy with my glass selection so don't plan on any near term investments there. The glass I have - given my use cases are fine.
I shoot mostly landscape and wildlife (really birds....). The D50 is more of my grab and go camera..while the D200 is for shooting things I plan to print/frame/etc/special occasions, et al.
I sometimes end up in low light situations - cloudy days shooting birds, or in the high school auditorium...
So my needs are decent shots up to iso 1200-1600, fast autofocus for birding, and of course decent dynamic range.
My plan is to keep the D200, probably sell the D50 (maybe).
The current thoughts are:
refurb D300 or D300S refurn d7000 or d7100
I really like the way the D200 feels in my hand and the features, but then again, the D7xxx image quality...
I am sure this has been beaten to death in the forum - but my thoughts are leaning toward the 7100. it seems that the additional features of the 300 and 300s (over and above the d200 i have now) is not worth it.