Kit Lens = Sucky (soft and slow)
But it also depends. You planning on doing any low-light, no flash concert photography? If so, you're going to need a FAST lens. FAST = $$$$. However, one alternative, is to go fo a "prime" lens with a fixed focal length. I think you can get a canon fixed 50mm f1.8 for seriously $79. It feels like a toy, but it takes super-sharp images a few stops up from f1.8, and then you have the fast-as-hell f1.8 with the XTi's ISO1600 for those no-light, no-flash moments of need. But, hey, that's only one lens, and one without zoom, and with the crop of 1.6x on the rebel, it's like really having a 80mm, so forget about wide shots. You will shortly need another lens. But you already know that. The thing is, that kit lens starts at f3.5 and goes up from there with zoom (I think). That's probably not going to cut it for low-light, no-flash -- but not sure you care? Anyway, that's one option...