I remember when looking at the spec sheets for the 20 and the XT that they
had a small difference in pixel count. (small enough that they both end up
being a nominal 8 megapixel.

The slight difference in pixel count leads me to believe they dont use the
same sensor in both cameras.

I snipped this from dpreview.com

It's interesting that not only does the EOS 350D not use the same sensor as
is used in the EOS 20D but that Canon deliberately designed the pixel count
of the 350D to be less than its bigger brother. Despite this there's really
no visible difference between the performance of the two in this test. You
could easily perform a blind test and not know which crop is from which
camera. Remember that both also use DIGIC II hence identical tonal and color
response.

Different sensor sizes, giving you a 1.2 multiplication factor on the 20D
(so I've been told), -v- 1.6 on the 350D.

Not sure about processor speed, but the 20D can burst up to 5 FPS -v- 3 FPS
for the 350D.

The 20D also has a shutter rated at 100,000 shots -v- 50,000 for the 350D

Check out
http://www.bobatkins.com/photography/digital/eos_digital_rebel_xt_vs_20d.html

for a comprehensive review from someone who's probably forgotten more than
I'll even know about photography.

Cheers,

CC