Here's the Nikon lineup, from high to low. I've slotted in the non-current models from your list as well.
Professional:
* D3X
* D2X (recently discontinued)
Semi-pro:
* D300 (6 frames per second, fast auto-focus, etc)
* D200 (recently discontinued. 5 frames per second)
Entry level:
* D80 (cameras in this segment are roughly 3 frames per second. If you're interested in the D300/ D200 you might also be interested in using prime lenses such as a 50mm f/1.8. The D80 is the only body in the entry level segment that can auto-focus with these lenses.)
* D60
* D40x (recently replaced by the D60)
* D40
The D40 is plenty for most people. It will produce magnificent full-page prints with practically the same image quality as the $1800 D300 and the $5000 D3x.
Put a $1000 lens on any of these cameras vs. a $100 lens on all of the others, and what ever camera has the $1000 lens will produce the best image quality.
All of these cameras are also very responsive (no shutter lag), and great in low light.
The price difference between the various models is justified by other things, such as more megapixels, faster auto-focus, more frames per second, better build quality, more buttons to change settings on the fly, and so forth.
The trick is to decide which features you're willing to pay extra for.
I personally bought a D200 because it was the cheapest model that offered 5 frames per second and a metal body (I don't baby my equipment).
Here's a side-by-side comparison of the specifications for all of the entry level and semi-pro models:
http://www.dpreview.com/reviews/compare_post.asp?method=sidebyside&cameras=nikon_d40%2Cnikon_d40x%2Cnikon_d60%2Cnikon_d80%2Cnikon_d200%2Cnikon_d300&show=all
You can also use this page to jump to in-depth reviews for all of the models.
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added:
Anthony, whoops, I got carried away there.
Please invite me over some time to toy around with that D3 :-)