The kit lens is actually pretty good. It's not great, but I used it and never regretted it. I would recommend using it until you get an idea of what to buy next. It will allow you to experiment without any more investment. When you start to feel its limitations, then you'll know where to invest your money.

By the way, I love the XTi. I have had lots of cameras and my XTi replaced an EOS 10D. I also have the original Canon EOS-1D, which I paid a fortune for. I just shot a big mountain bike festival and never once took the 1D out of my camera bag. The XTi is a camera that lots of people really underestimate.