out of the three you have listed - the 20d hands down. If you can afford the 30d though, get that. If you want to save some money on the body and get another lens, you can't go wrong with the Rebel XT. (don't bother with the canon kit lens)
I wouldn't bother with Olympus. Anyone looking to get a digital SLR I would recommend staying within the realm of Canon or Nikon.