I have the Canon Xsi and love it. Reading a few introductory books before buying is a great thing to do. Your initial decision on which brand to start with is important as it means investing in that manufacturer's lenses, flashes and accessories. These things cannot be used on other manufacturer's equipment, with a few limited exceptions. If you decide later to switch from Canon to Nikon or vice versa you are pretty much starting over. This isn't necessarily all bad, since the good gear that Canon and Nikon make resells well, but can be a big hassle if you have become accustomed to a certain manufacturer. I've heard some say it is like switching from a PC to Mac. And of course, in this case, Canon is the Mac.
I don't know how Nikon works exactly but make sure whatever camera you get can use all the lens that that manufacturer makes. If it won't accept them all then you are limited in the future. In the case of the Canon Xsi, it is not a "full frame" camera in that the sensor is not a 35mm sensor (it is smaller). Special lens were introduced for this camera called EF-S, versus the EF lenses for full frame cameras. So, as long as I buy EF lenses I'll be able to use them now, on my Xsi, and on any future camera I may buy. Some manufacturers don't allow their lower end SLR's to use the same lenses as the higher end SLR's, this limits upgradability.
For a better explanation of this see this link:
http://photonotes.org/articles/begin...enses.html#efs
Happy Shopping! Now is a good time to buy, I've seen a number of good deals on cameras amongst all the post-Christmas sales.