It also depends on your wants. The machine gun approach to shooting sports isn't for trying to get one good shot out of 10, but to catch that moment where the person has the perfect face while catching the perfect action. If you just want to get a few good shots then you could go with the XSI or T1i. If you want a faster shooting camera with better focus tracking then look at the 40D or 50D. If you want the best Canon for sports, without breaking the bank, then look at the Canon 7D.
You'll also have to figure in lens. Are you shooting indoor sports(85mm F1.8-$350), daytime sports(70-300mm USM- $550), or evening/night time/ decent indoor lighting(70-200mm F2.8 Sigma-$750 or Canon without IS-$1100). Buying the right lens can be expensive.
I am looking for a good Canon digital camera for taking sports pictures from the sidelines without costing a fortune. I would like changeable lenses for zoom ability.