Which Zoom telephoto?

[Zeca]

Hello friends!
I´m planning to by the Canon Rebel XT and I would like to buy a nice, but cheap zoom telephoto lens to shot birds, nature, etc.
I see that I have in fact three options of brands and 6 different lenses for the range to 300mm:

Canon:
1- Canon Zoom Telephoto EF 100-300mm f/4.5-5.6 USM Autofocus Lens
2- Canon Zoom Telephoto EF 75-300mm f/4.0-5.6 III Autofocus Lens
3- Canon Zoom Telephoto EF 75-300mm f/4.0-5.6 III USM Autofocus Lens

Sigma:
4- Sigma Zoom Telephoto 70-300mm f/4-5.6 APO DG Macro Autofocus lens for Canon EOS
5- Sigma Zoom Telephoto 70-300mm f/4-5.6 DG Macro Autofocus Lens for Canon EOS

Tamron:
6- Tamron Zoom Telephoto AF 70-300mm f/4.0-5.6 LD Macro Autofocus Lens for Canon EOS

I don´t know what is the best option, and I am a little bit afraid because it doesn´t have any stabilization and I don´t know if it will be a real serious problem to me. If it is a serious problem, could be better to wait and spend some more money to buy something better?
Thank you for your help!
Regards,
Zeca

[Clyde]

4- Sigma Zoom Telephoto 70-300mm f/4-5.6 APO DG Macro Autofocus lens for Canon EOS

This lens (actually the APO II?) will probably emerge as the consensus choice in this range. Yes, it lacks IS, but for what it is, it is well regarded.

Clyde