It also depends entirely on from where you plan on shooting the event. In the case of Formula One, dependent on the track and whether you're in the grandstands, behind a fence, or such, a 300mm zoom, and possibly a 1.4x teleconverter, should be sufficient.
Many pros shoot at the tracks with full-frame cameras at 300mm and 600mm on ISO 50 and 100 Fuji Velvia, and only occasionally with a teleconverter. With a 350D (1.6x crop), a 300mm zoom, and a 1.4x teleconverter, you can cover an effective range of up to nearly 700mm of 35mm equivalent, which should be more than sufficient for most any shot.
I got my girlfriend a subscription to F1 Racing magazine (UK) (and yes, I'm a very lucky man), and at the front of each issue, they have a 'Parade' section with some very impressive photos, and all of the shot information including lens, aperture, and shutter speeds... it's a great way to see what the pros use to get different effects.
/Andrew
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