Realistically with nikon and canon it all comes down to which camera fits your hand better, specs wise they are for the most part pretty much on par and you wont really notice any difference in the pictures.
The best bet is to look ahead to what you want to do in the future, when buying a modern dSLR its more buying into a system than buying an individual camera. You should typically spend more on lenses than on the body as a good lens will last you through several bodies with ease. There isnt much point having a 10mp camera if you stick a really cheap lens infront of it that cant resolve that high a resolution!
I dont know a great deal about the nikon side of things but on the canon side you have canon's flagship '
L' spec lenses, they are notoriously high quality producing insanely sharp pictures.
eg:
These two where shot with my '
L' spec canon lenses at an event last summer.
Canon 17-40 f4'L', Circular Polariser
Click for larger view
Canon 70-200 f4'L'
Click for larger view
Me Personally, I would go for the 400D and depending on what you want to shoot try and save up for another lens. At the start of last year I bought a 20D and worked off the kit lens for a few months untill I could afford the 70-200 f4'
L' then saved another few months to get rid of the kit lens with a 17-40 f4'
L'.