Hi LMSPS,
Haven't seen the photo yet but whatever you do, make sure your camera has a manual focus.
My Canon EOS3 has both auto and manual modes and I use the auto for some pitlane stuff and to preset the focus but that's about it.
Choose a marker point, and autofocus on that (or focus manually), then set to manual. If shooting head on, use a high shutter speed but if panning try for something a little longer to create an impression of speed.
I use a utility 35-350 Canon lens for everything. I know that CA Grand Fromage has much bigger lenses (compensation Dave???) and produces far better results than I can dream of but budgetary restrictions come into play here.
One bit of advice that John Brooks gave me which I think is invaluable (and assuming you have the facility available) is to fire three shots in quick succession just before the car hits your marker and theoretically at least one should be a goodie. This can obviously be a little expensive but what cost a few rolls of film for a cracking shot as opposed to a succession of "just out" shots?
Good luck!
Cheers
Rick