New lens help

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

As an olde pharte who has trouble holding a 400mm lens steady - yes.
I can hand hold with the 100-400 IS, turn IS off and it's impossible.

Your budget is the important thing though, maybe a monopod and the non IS lens will get you better quality for the price.

I'd ask how about non-Canon lenses? I found the Sigma 80-400 image quality good, apart from headlight flare at night, but the AF motor on it was slow.
At £890 is it out of your budget ?

How about a Tokina lens?
I've heard good things about some of them, what about their 80-400?
Only £360-400 so at 1/3 the price of the Canon or 1/2 the price of the Sigma - how does it compare?