Current used D200's over at B&H are going for around $930. A lot of it depends on condition and number of shutter actuations (sp???). A new D200 is around $1,300.
New D300, body only, is $1,800 at B&H. If you can get a nice, clean D200 for $800 it's a good deal.
I've seen some guys off-loading D200 kits on the Nikon boards for $1,200 that include a lens or two, extra batteries, CF cards, etc. Just gotta hunt around.
But if you have $1,800 to drop on a camera body alone then by all means get a D300. I love my D200 and will have it for a LONG time before I can even think of going to the 300. By the time I'm ready to do that they will probably have a D400 out!