Amy,
Are you planning to stay with Canon equipment? If so, then why not pop down to your local camera shop (if you have one) and try some/all the following; 1100D, 600D, 650D (being replaced by 700D), 700D and the 100D.
Of these, the 100D is the smallest & lightest - apparently it weighs just over 400g including battery which is lighter than your 400D. The 100D, along with the 700D were released earlier this year.
You don't mention anything about lenses so I'm not sure whether you'd be looking for just a camera body or one with kit lens.
Best of luck.