All you wanted to know and then some.
http://www.clarkvision.com/imagedeta...ary/index.html
After you have a look at the informative graphs head down to the tables at the bottom of the page. But basically the 20D has slightly greater dynamic range with deeper pixel wells, slightly higher f stop range, higher gain (e-/ADU/ISO) which will give less grainier images for any compared ISO, thought the 20D and slightly higher read noise.