Pretty much anything is going to be fast enough for you, I've shot motor racing with a manual focus film SLR, the trick is simply to find a spot where you have a good angle for cars coming together (I particularly like areas where a hairpin turn opens to a long straight, you get great shots of cars bunched up coming out of the corner) and lock in your focus and metering settings. All that's left then is to push the shutter release.
Any SLR will focus faster than the S3IS, but I'll agree that Canon has the fastest available.
That said, here's what I've ordered for my step into DSLRs: Sony A100 (body only: $799 from Adorama) and the Tamron 28-80mm f/3.5-5.6 Aspherical and 70-300mm f/4-5.6 Di LD lenses ($220 as a package together). It's not going to match the $500+ lenses that have been mentioned here, but I'm willing to bet that it would still be more than fast enough for your needs.
Of course, there's nothing at all wrong with the Canon kit either, go try both and see what you like. In the end, the "best" camera is always the one that you like. The best specs in the world are useless if you never want to use the camera, and I've seen some incredible shots taken with a plastic Holga.
Try shooting all manual focus a few times too, it gives an interesting perspective on taking pictures. I know that when I went from a nice AF point and shoot (film) to my SLR system, I found that the "loss" of AF really made me think a lot more about the shot before I took it, and I like to think that has helped me take better pictures. (Of course, better than crap is sometimes still crap.

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ZV