Matt pretty much nailed it. The only thing I would add is that when you are starting out as a general rule IMO you should steer clear of all 2x telextenders as they tend to degrade image quality noticeably more than 1.4X TC's. Common brands for teleconverters; Kenko, the least expensive and they make a few different models. Be sure to get the best one they make if you use this brand ($200 ish). Sigma $200, Canon $280. The Canon has lens coatings the others do not have so the IQ is better but how much better is subject to the users experience and the other equipment in use. The Kenko is very popular. You want to choose a TC after you get your lenses or at the same time as some brands have compatability issues with select lenses of different manufacturers. In terms of pure IQ your wish list (not considering cost, lenses listed are to illistrate my point only not reccomendations) in very general terms would run like this;
1) Prime lens. IE 400mm f5.6
2) Zoom Lens IE 100-400 f5.6
3) Zoom lens with 1.4 TC (+1 f stop)
4) Zoom lens with 2x TC
5 Lens with stacked TC's (subject to compatability of both camera and lens)

Most wind up using a zoom plus a 1.4 TC due to cost and flexability. A zoom lens will see more use for most. In the future you can then expand your lens collection if you want/need to based on what you like to shoot.

What you didn't tell us is what is going to be your primary walk around lens. I suggest getting the XTi kit with the 18-55. This lens gets a bad rap on the internet but I really like it. It only adds $70 or so to the body. No IS, NO USM but it takes great pictures and has a very close focus. I use a Canon EFS 17-55 IS f2.8 which is around 1k and I still use this little kit lens from time to time. Here are a few samples from the XTi and the kit lens. All hand held. Minimal PP, No sharpening or madd photoshop skills applied. smile


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BTW, It was the people on this forum that really started me on this photo venture. A few years ago I had no idea how to even post a photo. The XTi is a fantastic camera for the $$$.