ive just actually recently sold my d80 and went for the d90 when it was first announced. (if you can grab the d90)
The D80 is somewhat of a middle weight camera (or "prosumers") where the rebel XSI SLR being more of the novice and low end SLRs - it main direct competitors being the Nikon D40/60. They really in different class imo. From my experience The D80 was an awesome camera, and highly recommended, but like i said if you can go one up to the d90, where you get much better noise controlled pictures (due to the new CMOS picture) among other perks (live view, auto dust reduction, 720p video record etc.)
Like what zgnoud said, the XSI isn't the rival of the D80. I'd probably say the Canon 40D would be the D80's competition. I also sold my d80 right away and bought the d90 when it was first announced. If you have the money you can go for the D90.