I have a 300D and a 10D. I bought the 300D first and then the wife got into photography and sharing sucked, so we got the 10D. I was going to get another 300D, but the prices had gone down and a 10D was the same price as the 300D when I got it.
Anyway, as far as image quality, speed and everything else that usually matters you won't tell the results apart. In practice the difference is minimal, and the only real difference is the weight and the fact that I can't use my kit lens on the 10D.
There is the better build quality of the 10D. It feels much better in your hands, which one could say makes photography more rewarding. That in itself could be reason enough, but its a personal choice, how much does it mean to you?
I always choose the 10D over the 300D unless I want to use the kit lens. My girlfriend on the other hand preferes the lighter 300D.
Another small matter is the extra few functions. Mirror lock-up and the like. Again, they are down do how useful each use finds them. For some MLU would be a must. (You can hack the 300D to have most if not all of these extra options, the only downside being warranty issues.)
Conclusion: If you are happy with your 300D and it suits your needs, forget other bodies, your money is probably better spent on lenses, a good tripod, etc.
If you do find the 300D limiting and you have the money, I'd say with the 30D out its a good time to get a 20D. (Provided what you find limiting in your 300D is addressed by the 20D, like speed. Don't do it just for the pixels because on an A4 print you won't notice the difference.)