I can tell you from personal experience that megapixels is only a fraction of the equation, I still shoot on 10mp bodies, and see no need for more at this time. I still occasionally shoot a 20d for backup which is 8mp and with the right glass it still takes amazing pictures. The XSI is a nice body, but you have to ask yourself what kind of shooting are you planning on doing.
Things to consider are: Glass you'll be using, if you want a more "professional body", what image processor each body have (even if shooting raw), AF system's in each body, if you need video or not (my personal opinion is video is great but I'm buying a camera to shoot photo's not video myself so I'll gladly give up video to have other features) these are just a few of the things to think about. Personally I'd say go for a 40d or 50d and some nice glass and you wont regret it!