Yeah, I think I would agree with @timtam, the d300 is definitely newer technology and probably worth the extra $$. (It uses a CMOS sensor instead of the D200’s CCD sensor) Here’s an excellent comparison between the D200 & D300.
One question I would have … How much is the price difference between the two? Do you know what the prices are, and know whether it’s worth the $200 (or whatever the price difference is) for the more modern camera?
The D200 seems to be running about $200 (body-only) and the D300 is about double that $400-$450 (body-only). From a quick look on ebay, it looks like the you might be able to get $150 (body-only) for your D50.
My brother is a Pentax fan, and he has mostly won me over to his position, (I now own a Pentax K-5) He would suggest that you buy the Pentax K-5 IIs which is $489.00 (Brand New, Body-Only) on ebay. That’s only about $50 more than a used D300, and the Pentax is several years newer, which puts it far ahead, as far as specs are concerned.
Of course, if you’ve already got a large collection of Nikon glass, it may not make sense to switch over systems, but it’s worth considering! 🙂