I've used my mother in laws d40 and liked it well enough.. though i didn't care too much for the MF. I like the size it might have been a little too small?? I havn't had a chance to hold a d80 yet but from what i've read so far it seem most people say spend a little more and get the d80. One thing I didn't like about the d40 was the lack of the settings window on the top of the camera.. as this is what i'm use to.
The way to deside is to look at your budget. How much will you be spending on Camera and lens this year? Buy yhr camera body that alows you to spend half your budget on lenses. The D80 with jjust the $100 kit lens is a waste. If you are going to spend $1K on a camera don't do a $900/$100 split try for a $500/$500 split Over all you will get better reults with an SLR system if the mmajority of the money is spent on lenses. But I'm talking not just the initial purchace but, say the first year or so.
The D40 is designed for some on who will buy the 18-55 kit lesn and then maybe one of the cheap f/5.6 zooms and that's it. the D80 is targeted at the person who wil be building a larger system
I's not good to think in terms of "which body is best" but rater "how shall I best allocate my fixed budget so as to maximise reslts" I think the D80 is good for the person who plans to spend $2K on the SLR system in the first year or so.