I have a 300D as well. The hacked firmware really makes this camera worthwhile, since you get ISO 3200, other options, and mirror lock.... and there are several second timers that you can use with it, starting at 1.5 seconds! The only thing is that you need to suck it up and risk turning your camera into a paperweight, but I've had no problems with mine. With the a hacked 300d, you could afford some really nice lenses for it.
Also, from what I hear, the 20d and XT image quality isn't a significant improvement from the 10D/300d, it's only a matter of more megapixels, slightly better in-camera noise reduction... and faster RAW write speeds.
20D would be the best as far as build and general performance, but strictly speaking of XT or used 10D, I repeat my suggestion for the XT!