As Walt/Scott have posted I would invest $ in lenses first and bodies second. You're not going to see major technology changes in lenses the way you will with bodies over the next few years. You will undoubtedly get more keepers with the 20D over the XT initially but as your skill level goes up along with that comes an understanding of how different the image quality is between an L lens and a consumer lens. Your collection of lenses will always be there for you while the bodies will come and go. I've seen pictures I could only dream of taking on the photo forums from the Rebel but they were usually done with a top quality lens.
It's good to remember one other thing as well, 1.6X crop cams like the Rebels, 10 & 20D only use the center portion of the lens and hide a lot of the image quality issues that will show up on 1.3X or full frame imagers. And Canon has expressed a committment to lower cost FF imagers with the introduction of the 5D. A lens that looks great on a 1.6X crop imager may not be so great on a FF. When you buy an L lens you're paying for reasonable image quality across the full plane of the lens. So it's nice to know your lens collection won't turn to cr@p
if you end up with a better body. I think (hope) Canon's next offering will be a 1.3X crop sports body, 10-11mp with 1D speed/AF priced under $2000.