I own a 400D and the build quality is not bad at all. A friend of mind owns a Nikon D80, build quality very good.
As for picture quality, about the same really. So I would love a 40D, how ever I am going to stay with my 400D for a few years to build up my skill then perhaps chop it for a 40D model.
Also I find decent lens are very important, I have a Sigma DC macro 17-70mm which I think is a cracking little lens. Say little is actually some what heavy !
The whole 40D question is I think, would you use the extra features it can give you ! ?
Is it portable to go every where, ie holiday, indoors, weddings etc.
I recently took my 400D skiing with me as it fits easily into a small bag.
convience is just as important as features !