the 20d was an excellent camera in its day, and is still a very good camera capable of outstanding images. i owned 1 for about 5 years until here recently upgrading to the 50d.
i did a quick check on ebay and such, and the price of a 20d is not that much different than a new Canon XSi with kit lens. the XSi is several generations newer, and while of the lower rebel line, it outperforms the aging 20d.
i think the XSi would be a better and safer purchase at this point. many of the 20ds are getting older and may have many shutter actuations. and the shutter button or shutter could wear out, which is in fact what happened with my 20d.
if you know the person selling the 20d well, such as a friend or relative, and you know it hasnt been shot much and has been handled well and you can get a really good price, then it could be an option.