Once you have used the 5D, all your lenses take on a different meaning. The viewfinder is another asset. I just got rid of my 10D for the 50D and it is taking a while to get used to it. As you say, keeping the 40D will give you great options. I am finding I am using my 50D a lot more than my 10D with the 5D around. I mainly use the 5D now for studio work.
Btw guys, I have been hunting for a 5D for days but unable to score one locally. Those waiting time has made me reconsider my options, and somehow I ended up with a spanking new 5D MkII. I started thinking about getting a used 30D, internet is a dangerous place.
