The Xsi is a fantastic DSLR for a beginner and is the quintessential tool for someone just getting into photography and wants to go with Canon's line of lenses. Even the kit lens on the Xsi make for great photos, which is surprising considering all Digital Rebel models before the Xsi came with very poor kit lenses. The only thing that's different between the Xsi and the 40D is slower continuous shooting and somewhat poorer ISO performance on the Xsi's end. The 50D has better IQ at 100% magnification and is light years ahead of the Digital Rebel line in terms of ISO noise performance. The **D line also has a more professional build quality associated with it, though I feel this is more of a cosmetic/placebo effect for a photographer since, if you take care of your body, this aspect is irrelevant. The Xsi feels good in your hands and has a nice grip. With a good lens, you can get professional quality photos with it.
That said, years down the line if you do decide to shoot professionally or you want to take your photography to the next level, consider getting the 5D line so you can take full advantage of a full frame sensor, as well as stellar build quality, phenomenal ISO handling, 1080p recording, and who knows what else will be around for this full frame line by the time you choose to upgrade. I see no point in getting the **D line now that you've gotten an Xsi, unless you find that you're primarily shooting action and absolutely need the better continuous shooting. As for the 1D line, I would only consider getting one of these cameras if you are actually making money off of your photos and are making a career out of this, or if you're simply stinking rich.
Long story short, you made a good decision. Now it's time to become accustomed to photography, get some solid lenses (Do not cheap out on these is my best advice), and decide where you're going to go with this. The Xsi will likely keep you satisfied for a long time if you're just doing this as a hobby, and if you decide to go semi-professional, it can even still serve as a good back-up due to it's nice size and portability
EDIT: Also, if you want to see my photos, they're up on
http://alexpylyshyn.blogspot.com I am currently limited to the kit lens however.