the d80 is way out of the league of the other 2 i think.

but anyway, this is what i think about them.

the d40x,
its a really good camera for beginners i think, cos got a lot of information, built-in help, abit smaller. but the only thing is that it doesn't support autofocus on lenses other than AF-S or AF-I lenses. and it also doesn't have AE- braketing. so, if you currently have some AF-S or AF-I lenses, no problem on manual focusing, not doing HDR photography, have a tight budget, you may consider the d40x.

the d80,
although it's made out of plastic, it feels really tough. i've tried it b4 and trust me, the ergonomics are fantastic, at least for me. it feels nice to hold on, but its a little on the heavy side compared to the other 2 but you should get used to it eventually. to be short, this camera has more than everything a beginner and a hobbyist need, i think. if you have the budget to get a d80 and since you are currently using a d50, why not get this?

the 400d,
i've never tried this cam b4 but from what i've seen in the reviews, its more or less the same with the d80 only that it weight abit less, measures abit less, use different memory cards, dun have a secondary lcd, and most importantly, it has a sensor cleaner. in fact the only thing not so good abt this cam is that i've heard alot of ppl complaining abt its kit lens.

canon and nikon,
well, by comparing a lot of samples taken by canon and nikon dslrs, i think that i can come into conclusion that canon takes razor sharp pictures across iso ranges, good for sports i think while nikon manages better with colors,good for portraits and landscapes i think. so in the end its really up to what you emphasis more on.

the 6mp to 10mp jump,
in practice, it doesn't really make much of a difference unless you are printing poster sized prints. if you're just planning to have a 4x6 print with 300 dpi, 6mp is more than enough.

if mp count is really that matters, my recommendation for you is that since that you are considering the d80, currently using a d50 and might have some nikon lenses, why not get it?

this is just my recommendation to you. get more advice and then consider more.