The lens with the IS is super expensive. If you have a steady enough hand it will not matter. You can pick up a 70-300mm lens for around $200 without the IS function. Go ahead and get the XSI. The best part about the Cannon Rebel series is that you can just use the standard setting and still get great shots.
Also the features of the camera are important, but knowing how to use your camera is even more important. Check out this website, it is very informative.
www.kenrockwell.com he goes into using the different settings TV, AV etc..
I own an XSI and love it. There are also more aftermarket lenses for Canon cameras than there are for other brands.