You need to worry about 2 things: focal length and maximum aperture.
Focal length: Prime lenses (fixed focal length) tend to be more cost effective while delivering optimal performance, but you will probably want to stick with zoom for sports. Focal length needs can change constantly during the game. With XT/XTi's APS-C crop sensor, 70-300mm tend to be ideal for sports (70 = 112mm, 200 = 320mm, 300 = 480mm).
Maximum aperture: if you are shooting with limited amount of light (e.g., night time game, not-so-brightly light indoor game), you will want all the aperture you can afford, ideally at or faster than f/2.8 across all focal lengths. Popular examples include Canon EF 70-200mm f/2.8L USM ($1100-1300) and and Sigma 70-200mm f/2.8EX ($800-1000).
If you will be shooting against good lighting sources, or don't mind shooting at ISO 1600 most of the time (although even 1600 may be limiting under some conditions), Canon's EF 70-200mm f/4L USM and EF 70-300mm f/4-5.6 IS USM are two very good choices, both just slightly above your price range.
If you don't have tripod, or can't carry one all the time, you may want to spring extra for image stabilized lenses (indicated with "IS" designation on Canon lenses), such as Canon's EF 70-200mm f/4L IS USM ($900-1200) or EF 70-200mm f/2.8L IS USM ($1600-1900). (Above mentioned Canon EF 70-300 f/4-5.6 IS USM is also image stabilized.) As you go longer in telephoto shots, it becomes harder to hand hold the camera for steady shots against limited light.