I have to be careful how I phrase this as I don't want my 40D to die on me but even if I had to begin using the 400D again tomorrow this would still be the last picture posted that I've taken with the 400D fr quite some time. :-)
Please don't expect any dramatic improvement in picture quality! On the contrary, it's taken me a while to find my way around the 40D and there was also a downturn in the weather just when it arrived.
The best things about the 40D as far as I'm concerned is that flight shots have become much easier - AI Servo actually works and indeed I use it most of the time. And lower noise at higher ISO means that I can get better pictures in low light conditions.