Hi
Sounds like the Canon 70-300 f/4-5.6 IS would suit you down to the ground. Costs about $600 and a review can be found
here It will have plenty of reach for you, and the IS is a great feature which will really help your keeper ratio. Sure, it is a little slow (i.e. doesn't let in much light) but you can compensate for that by cranking up the ISO in low light condition. Also it is a little bit soft at the long end, but not too bad.
You could of course get a better lens which also fits your requirements, like the Canon 100-400 f4/5.5L IS and you will get better image quality (but not more speed). But at this stage I wouldn't recommend that. Firstly, a better lens will cost you more (a lot more). Second, it will be bigger and heavier (much heavier) and will not balance well with a tiny little thing like a Rebel XTi. Thirdly, these big lenses simply scream "look at me, I'm a pro" which you may not want at this stage.
Hope this helps. Best of luck with your learning. Enjoy.
Ed