A friend of mine has the 350D (Rebel XT). He's thrilled with it. I played with it and was struck by the awfully tiny hand grip. The optional vertical release grip feels wonderful, but isn't all that useful when you're shooting horizontally. I've never played with a 20D in person. The faster frame rate would be moderately attractive, but it's really your call on whether you want to spend all that extra money.

Going with an older-generation pro body is an intriguing choice. The number one question you should be asking (assuming you're buying used) is how many clicks are on the shutter. Usually, pros don't give up their cameras until they're worn to pieces. Beyond that, the main issue is simply the weight of the damn thing. If you're carrying it around your neck, you'll feel that weight in a hurry. I wouldn't worry too much about the megapixel business. Just ask yourself whether you do much in the way of cropping. If you like to cut out pieces and call that the picture, then the additional resolution is worth keeping around. Or, if you anticipate making larger prints (16x20 or bigger), then you likewise might prefer the higher resolution.