we've got the 40D and the Sigma 70-300mm lens is what i use most of the time for motorsport photos (all of my rallying pics etc were taken with it). Also got the 18-55mm Canon kit lens that came with the camera, but tbh its pretty poo and only use it for taking pics of static stuff
The Sigma 70-300mm is very good for the price (i found it for £99) but its not up there with the pro stuff. But then neither's my budget
If you pay a bit more you get better quality (obviously) but you get extras like Image Stabilisation and stuff like that. The cheaper lenses also struggle in low light conditions more.
You're welcome to have a go with our 70-300 at a meet sometime if you want.