From what I've read on the D300, if I was in the market to upgrade from my D50, I'd go with it. Cost is approximately $1800 for the body. Right now, I'm more interested in new glass.
http://www.kenrockwell.com/nikon/d300.htm
...but like PaddyJoe says, it's a good idea to see if your old lenses can work with a new digital body, especially if you have professional quality stuff.