The 40D is a far superior camera but as said previously, image quality is about the same on all of these. It is the extra bells and whistles which make photography much easier (and imo more enjoyable as a whole) which set them apart.
Now I do a lot of auto racing, football, basketball, volleyball, and other sports. The 7 pictures per second with 17 RAW or 75 JPEG in a row for example makes life MUCH easier. If your REALLY good you can take just one picture with an Xt and be solid. I anticipate a play and go ci-ci-ci-ci-ci-ci-ci-ci-ci-click. Keep the 3 good ones and throw away the rest. There are many other examples like this which set cameras apart. Image quality is marginal.