Go to a store and hold both of them in your hands.
The 400D has a much bigger screen, and only one screen which shows the settings rather than an extra, small screen for the settings. The 400D has an eye sensor so it know when you hold the camera up to your eye (to shut off the screen) The 400D has better access to the settings via buttons on the back rather than going into the menu.
The 400D has the vibrating sensor to get rid of dust, and it also has the dust spot elimination setting. I'm betting that all new Canon DSLR cameras will have this feature.
IMO, the new 400D is worth the extra $200. It's actually a better camera, in many ways, than the more expensive 30D.