If you are shooting indoors with ISO 1600 and aperture 5.6 your shutter speed might end up being less than 1/100s for a properly exposed photo. If the lighting is poor enough your speed might be in the order of 1/40s. That is too slow to stop motion but there is no way around it if that is the limit of your equipment.
The ISO, aperture, and speed work together to get a proper exposure. If you double the ISO, you double the shutter speed. If you increase the aperture by one stop (which doubles the light entering the camera) you double the shutter speed.
Here's how that works. You start out with ISO 1600, f5.6, 1/40s. If the Rebel XT is capable of ISO 3200 (not sure about this) you could have ISO 3200, f5.6, 1/80s. If ISO 1600 is the highest ISO then you can increase the aperture one stop to get ISO 1600, f4, 1/80s. Increase the aperture one more stop for ISO 1600, f2.8, 1/160s. One more stop would give you ISO 1600, f2, 1/320s. That shutter speed (1/320s) is fast enough to stop a lot of the motion blur.
But the only way to get to f2 is to use a lens that is capable of that aperture. That is why a lot of people will use the 85mm f/1.8 lens for action shots indoors. The lens you have now is probably only suited for indoor still shots or indoors with a flash.
Does all that make sense?