Hi Ray, as I wrote that I thought you'd ask - and you did ; the bet and shortest response is to refer you to stuff Michael has written about the D3 and D300 on this site. Three distinctive features which come to mind are the auto-adjusting ISO, the easier route to mirror lock-up and the very high ISO (with surprisingly clean results at those levels of sensitivity).
As for the 40D, of course it's always good to have a second body for insurance, and the higher frame rate could make it particularly suitable for some kinds of work, but that 5D remains a very fine camera. From what I've seen of it, unless one really wants more MP or those added features of the newer models, it's still a keeper.