This is a good link:
http://www.bobatkins.com/photography/digital/eos_digital_rebel_xt_vs_20d.html
Main difference is the 20D is bigger and more substantially built, and has a better shutter that can run at 5fps. It is also quite a bit more noisy, which might be a consideration. AF is said to be superior, but overall the 350D is a very capable camera indeed and undoubtedly the best value D-SLR at the moment.
The other big difference is the cost. You can get another nice lens for the extra over the 350D, which is what I did. Another consideration for me, and this is just crystal ball gazing, is that the 20D may be upgraded soon - perhaps at the PMA show in February next year. The current £100 cash-back could be a clue. Of course, the 350D will get a replacement eventually, but it's a newer camera so maybe not yet. Wait until Feb if this is a concern, but I seriously doubt that you will be disappointed with either camera. They're both superb.
Richard.