The 1100D rules bang for buck for ap work, but terrestrial they blow bad.
http://en.wikipedia.org/wiki/Compari...igital_cameras
20D digi 2 processor 8.2mp APS-C (22.5 × 15.0) CMOS
450D digi 3 processor 12.2mp APS-C (22.2 × 14.8) CMOS
1100D digi 4 processor 12.1mp APS-C (22.2 × 14.8) CMOS
garys T3 review sums it all up
if you can't get an 1100D on your budget then a 450D would be sweet as well. I got my last one at harvey normans for $398 with the basic 18-55 non is lens, so see if you can stretch your budget. Next up would be the 550D though i doubt your going to get a body less then $500, currently $550 at teds body only. The 1000D isnt bad though has higher noise levels but would be an option with your budget. The 20D doesnt support live view as far i know and also why the hell you would buy one over a 450D is beyond me, unless its for terrestrial work.
Do your home work anyhow and read the reviews for iso noise levels, also remember older models camera in the mid to low end have very short shutter life spans, so check before you buy