I think the Xsi is a fine camera, just as the XT you already have is a fine camera. The question is whether replacing an outdated camera with another outdated camera in the same class is worth a 6/7th price difference and missing out on the latest and greatest technology in that class. Personally if I was using an XT I would be looking into a 50D.
To someone buying their very first DSLR, I would happily recommend buying an outdated camera (even your XT). This is because the Xt can take a picture, and the lenses are where quality really shines. In the name of saving money for a better lens which would be bought at the same time, sure buy an outdated camera. If you are looking to replace an XT, you should already have a quiver of decent lenses so why buy another old camera? If you don't have a few good lenses I would be more concerned with covering the bases there first. For a current DSLR owner who already has a foot hold in the game, the only outdated camera I would seriously consider a worthwhile upgrade right now is the origional 5D. Then again that is just my personal opinion...but I think the added benefits of the T1i outweigh the 1/7 extra cost of the Xsi for someone who already has a camera meaning extra benefits over what you already have is the only point in this venture to begin with.
