IS is a good thing.. you gain up to 2 stops on the exposure.. hardly something I would call marginal when you need it. If all of your pics will be taken outside in the sun.. there is no need.. but in low light or indoors.. especially with moving subjects.. the IS is a good thing.
Now.. on to suggestions..
The 18-55 kit lens with the XT is crap.. dont waste your money on it.. the other one available as a kit is pretty good though (17-85 I think)
The 50mm 1.8 lense is a great bang for the buck.. sharp lens and very cheap (less than $100)
The 28-135mm IS is a very good every day lens (mine stays on the camera most of the time).. It is, in my opinion one of the best none "L" lenses that canon makes.
If you can afford it, I would also get a 70-200mm 2.8, the f4 is a great lense too.. but the 2.8 is one of the best lenses out there in my opinion (with or without IS depending on your budget)
The Sigma 70-200 2.8 Lens is a great lens as well.
If your plans include shooting lots of captive animals.. I would also look at the macro lens selection.
The biggest 2 things I can tell you that will help you with the Canon Rebels.. get a good book on photography and read it.. and get a flash instead of using the built in one..
www.dpreview.com has a forum specifically for canon lenses as well.. there are lots of debates on what is good and what is not always going on over there.. and there are usually a good selection of examples as well..
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