Nikon is so much easier to understand than Canon. At least that's what I found when I was looking at cameras 6 or 7 years ago. Take your 20D for example. What does the "Jump" button do? How about that Asterisk button? Whats the point of that one and why does it get its own dedicated button? Talk about confusing. The menu system is even more confusing to navigate. Everything is far easier to understand with the Nikon cameras, and if you get confused you can always push the question mark button and it will tell you what you are looking at.
I recommend something in the middle. Not total beginner where things are stripped out of the camera, and not full blown where there are more buttons than a QWERTY keyboard. I recommend D50/70/80/90 cameras.
I think I've owned or at least played with every body you mention. Depending on YOUR uses, I'd either go with the D200 or D40, or I guess potentially the D3100.
For low light use the surprise might be the D40. Last generation of 6MP bodies and they really got it nailed IQ wise. However, you do lose a lot of the external controls in comparison to the D200 or even to some extent the D70. D200 was one of my favorite bodies - however high ISO performance wasn't a strong suit. I put a zillion miles into a D70 as well, but they're pretty long in the tooth now, despite being solid IQ-makers under ISO1000. The D40 is diminutive, but keep in mind with it, the D3000, or the D3100, you are giving up an in-body lens motor, which could affect your lens-buying decisions. If you need the motor in-body to open up the lens market you're after, the D200 is still exceptional at lower ISO's, and built well enough to last. D100 I think I would avoid at this point. The D70 is much improved in many ways on it.
