Actually, the 20D does, in fact, have a higher resolution than the Digital Rebel.
As was noted, don't get confused by the 72 or 180 dpi numbers that get stuck in there by the camera. Those are fairly arbitrary number, and you can change them with any good photo editor. As an example, if you only publish images on the web, then you might want to convert them all to 72. If you only do very high quality inkjet prints, then you might want to convert them all to 300.
For years, I have used a different photo editor that does not require me to use any 72 or 300 number in there at all. I can print perfect prints to the paper size I set. In other words, it throws the entire resolution at the printer without me deciding anything. That works for me.
---Bob Gross---