Out of the XTi, XT, and 20D, I personally would opt for the 20D myself.
Better built, better ergonomics and control, and better lowlight capabilities.
Though, with your budget (is it $500 for body AND lenses?), I'd definitely take xarqi's advice and opt for the cheapest body, and work your budget around the best lens you can get for the type of shooting you want to do.
Just don't forget that you need to set some money aside for a camera bag, memory card, and perhaps just a couple other accessories (cleaning kit perhaps, filters maybe).
Cheers and good luck!
Frank