Stumbled across this website, I find it to be very informative when planning a trip to a race.
Includes circuits from F1, ChampCar, IRL, NASCAR, MotoGP, Superbikes, ALMS, AMA, FIA-GT, and a comprehensive state-by-state list of American short tracks, dirt tracks and speedways.
http://www.grandprixcities.com/series.html