I don't even know why the 400D was released. It wasn't much better than the 350D, if at all. The 450D was a better camera, if not because of the addition of spot metering (why was the most basic form of metering ever absent?), and a larger viewfinder. This only happened due to the new competition from Nikon, which wasn't an issue during the days of the 350D. There was no need for Canon to offer any real improvements to the 350D, so when "developing" the 400D, they didn't really give people more. In hindsight, it was a mistake, and the 400D was the product of Canon's mistake.
Get a 450D.