I think before everyone just blurbs out lenses, what sort of sports do you intend to take especially indoor ones? What type of shots do you want to take and how far are you away from the action? What is the use of a 50mm if you are sitting in the stands trying to capture the badminton game. Most probably you might just get full blown shots.

If you are using a Canon DSLR like the 30D, 40D you can benefit from the low noise at High ISO especially if you shoot at 1600 and above.