I was just wondering if someone might be able to help me? Ive recently purchased a Canon EOS 20D with the 17-85mm IS USM lens, and im looking to get a new lens for the up-coming motorsport season.

The lens im look at are -
EF 75-300mm f/4.0-5.6 III USM,
EF 75-300mm f/4-5.6 IS USM,
or EF 70-300mm f/4-5.6 IS USM.

The only thing really putting me off the IS lenses, is the fact they are over double the price of the normal lens, and essentially they have the same spec (except from the IS of course). Are the IS lenses worth the extra cash?

Any thoughts welcome.
Thanks in advance,
Oliver