Canon EF 85mm f/1.8 USM Lens for about $420 (less $ if used). 85mm on a your crop sensor may be enough if you can get close to the floor. The amount of light will be better too closer to the floor.
Have to agree with the above - indoor sports under $500...85mm 1.8 would be my choice. If going used, maybe you'll get lucky and find a "fast" zoom. Working on your focus techniques will go a long way as well. Mine is not quite there when it comes to shooting indoor sports.
