It's always nice to have something new and shiny (I am a sucker for this). When I compare the shots taken on my old 350D (not a big jump from the 350D to to the 400D?), the difference is startling. Having said that, I am sure a 60D or 7D would also be a huge leap from a 400D. One thing you may consider is that if one of your problems is not being able to focus on moving subjects, maybe it is the lens, not the body? I don't know enough about the 400D to comment, but maybe a faster lens would be of benefit? If you buy a good lens that can be used on a full frame body (like the 70-200 f2.8), you will be able to take it with you if you eventually upgrade to full frame.

The 5D MK II is still far more advanced than I am. I need to get a whole lot better before I consider upgrading. In reality, I could probably do nearly as well with a lower spec camera, but I don't drink, smoke or waste money on other things, so this was my indulgence. I also work hard, so guess I can spend my spare care as I please. Well, as long as my wife agrees!