You don't need an expensive camera for sports photography, you need an expensive lens. If sports is what you want to shoot, start off with a cheaper Canon body and maybe a 70-200 f/4L. Or since you've got some budget to play with, either a 70-200 f/2.8L with or without IS, with or without a teleconverter (1.4, 1.7, or 2.0x). A 300mm f/4 telephoto prime would be fun to play with and start off with too. The best lens sorta depends on what sort of sports events you go to, where they are, how close you can get, and the lighting conditions.
As far as the actual camera, just try them out and see what fits your hand best. Any of the Digital Rebels (XT, XTi, XSi) are fine to start off with, but also try out a 30D or the new 40D. Plan to spend more on the lens than the body, though. I'd pickout the lens you want first and then get the body based on what's left over. The 28-135IS lens that commonly comes in Canon kits is a nice travel lens, but not very wide. If I was buying a Canon I'd get it with the 17-85IS lens. And I say Canon in this case because Canon lenses generally have quicker autofocusing at the consumer level than Nikons, and Canon also has a much more complete lineup of telephoto zooms. And then their telephoto primes are often
thousands of dollars cheaper than Nikons. And most sports photogs all shoot Canon, so if you make friends with one and he lets you borrow a lens, there's a 99.9% chance that it'll be a Canon lens.
Sports = Canon.
If you're not sure if this will take off as a hobby for you or not, buy all your stuff used. If you decide it's not you you could probably turn around all the stuff for about the same price you paid for it, and then you won't be out any money.