I've had a D50 and a D70 (not the S) and now a D80. My recommendation at this point would be to buy a D50, then a D70, then a D80 in that order. If you're patient enough to wait the 40x might be good. Basically in my experience thus far I've had the best results out the of the 70, the 50 is the best compromise of features, and the 80 has the most features but the images aren't as good as the 50 or 70. My concern is that the 10 megapixel cameras from Nikon just don't seem to be doing that great a job.
I'd call myself an upper amateur, I understand every feature on the camera and when it should be used...or so I thought till I've used the 80. If you're a proficient amateur I'd stay in the 50/70 neighbourhood, you'll actually get better images than you will with the 80 or 200. Also, the 40x is pricey when compared to the two...save the money and put it into a lense!
The reason why the D80/D200 doesn't produce as good a picture out of camera is due to the fact that it doesn't fudge/process the pics as much as the others. Otherwise, the dynamic range of both of these will trump the D40/D40x/D50/D70 any day.
The D80/D200 is gonna weigh more than the D70. This weight is partially due to the internals (focusing motor; internal frame) and also the gaskets (e.g. weathersealing in the D200).