You should pick up a "
Thrifty Fifty" - Canon EF 50mm f/1.8 II Camera Lens for ~$90. The f/1.8 is a huge advantage over the f/5.6 on the 55mm on your kit lens, at least for indoors, or portraits
Other than that, you are probably best off listening to the other guys about buying what you need, not what you think you need. It's my opinion that if you do buy the IS version of the kit lens, you check ebay for someone who bought the XSi kit and is selling the lens, you could possibly get it pretty cheap that way (?). My opinion would be that you might want to get something at a telephoto range (200-300mm) just because you already have something sort of wide-angle (18mm).
I hope this helps!