I agree with Jimmy that Av is the best way to shoot at least 90% of the time. ISO can help control shutter speed at a given Av or of course changing the Av for shutter control if the DoF isn't important.
1/100s at high ISO seems very low even at f/5.6. Must be down to your camera? With a 20D I was using a Sigma lens which was f/5.6 at 200mm, and in very low light I was getting 1/220s at ISO 800. At ISO 1600 the shutter was easily getting around 1/500s.
So I think your problems are localised to your camera, and that for most people Av is the best way to shoot RC action. That said, I often use a low ISO with Av to get down to 1/100s shutter, gives very nice results
