The Nikon D40 is a little fussy on lenses, ie autofocus will only function with the newer AFS lenses and not older lenses, or with some third party (ie Sigma) lenses, where as any Canon eos fit lens will work on the 400D.

Canon 400D gives you 10 megapix, Nikon D40 is only 6 million...

Of the two cameras you mention, I personally would go with the Canon... LOL ... and I use both Nikon and Canon gear!