I used to own the Rebel XT and while it was a good camera I like my 40D better. The 40D feels much better as it is a bigger body and you can access most settings (ISO, aperture, shutter, focus, drive) quickly from dials or buttons which means you can change some while the camera is at your face rather than going through the menus. That said my brother just bought the new Rebel and he is very happy with it. Looks like the both the lens you mentioned are okay for a kit lens so it depends on what you plan on shooting. With the 1.6 crop factor 55mm is going to be like 88mm which may not be wide enough if you like landscape shots. If you only plan on shooting close up tank shots I can't say enough good things about the 100mm macro.
Edit: I just re-read your post and you are getting both lens and the body for $850.00? That sounds like a pretty good deal to me!
Ken