The XT, 20D and 30D, IIRC, all share the same sensor. While Canon claims some differences amongst the various camera sensors ... the difference., if any, are not significant.
I see no difference in IQ between the XT, 20D and 30D. While the 30D is clearly a superior camera in terms of features ... there should not be any visible difference bewteen the two cameras IQ wise.
In fact, my experience has shown that at ISO of 200- and at sizes of 8x10 and less, one cannot see any visible difference between any/most dSLR's, including the D3 and the 1DsMKIII. (We're not talking medium formats here.)
Gary