The difference between the 20D and 30D comes down pretty much to the 30D has a bigger (2.5" vs. 1.8") LCD and spot metering. It supports Picture Styles, but you can add those to RAW photos from the 20D in post-processing if you really want them. The 40D adds an extra 1.5 fps (from 5 to 6.5) in continuous shooting mode, has a 3" LCD, self-cleaning sensor, 20% more pixels (about a 10% increase in resolution) and Live View. How much any of these are worth to you, only you can say.
A word to the wise, though - if you're used to wide angle in 35 mm, you're in for a shock when you put your wide angle lenses on a cropped camera. They just aren't wide any more. This works to your benefit with telephotos, though it means across the board that you can't hand-hold at the same speeds you were used to in 35 mm. The cropped sensors give pictures that need to be enlarged more, and that means unsteadiness is also magnified.