400dabuser - once you get used to shooting a XXD, you will not want to buy a XXXD camera again.
Figure out what you want/need from a new body and decide from there. I decided the ergonomics alone were enough to make the jump (but there is the frame rate, high ISO etc as well)
The XXXD cameras will always be smaller, have harder to access controls (for ISO, WB, Metering mode, etc), have only one wheel (which makes it harder to adjust settings), the viewfinder looks so dark by comparison and so on.
The only issue if I were you is when and that has to be your call.
1) Buy a previous model (40D) and save some money (still a tremendous camera)
2) Buy the current model (50D) and enjoy the best Canon offering (in the price range) atm.
3) Wait for the next one (60D) and put up with the shortcomings of the 400D you have at the moment.
Having a camera makes you no more a photographer than having a hammer and some nails makes you a carpenter - Claude Adams
Keep calm and carry a camera!
My Gear