basically the 300D is the "rebel"...the first digital rebel released...then came the XT and most recently the XTi...also known as the 350D and 400D...
the XT and XTi are slightly smaller (slightly)...also I think MP went from roughly 6._ in the original rebel to 8._ in the XT...and then about 10._ in the XTi...
there are other differences as well, i think ISO capability gets a bit better....startup time, LCD size,
the XTi has a sensor cleaning doo-dad that cleans the sensor with micro-vibration every time you turn on the camera....
there's other stuff too...others will chime in soon... 
".....Nice camera! .....How many times zoom?!?"