May I suggest
www.dgrin.com as well. Its not quite as busy as fm.com and the folks there are a lot like the folks here...nice and willing to help a noobie in need.
The lenses I use most for concerts/lowlight shooting are my sigma 24-70mm f/2.8 (soon to be replaced by the canon L version) and the 70-200mm f/2.8L non IS (non IS was sharper than the IS version, and I like to think I have a steady hand). I use the 50mm f/1.8 on occasion in extreme lowlight circumstances. Plus its dirt cheap so theres no reason why you shouldn't have one.
A good rule of thumb, your shutter speed be equivalent to your focal length...so if you're shooting at 200mm, your going to almost need a shutter speed of 1/200 to get a steady shot.
The xt does just fine in low light from all the pictures I've seen, however, I've never used one, but for the most part its quite similar to the 20D and 30D just without the bells and whistles.
other good sites for reviews on lenses...
www.photozone.dewww.pbase.com - you can search lenses hear and get sample pictures taken with the lens by pbase users.
http://www.photodo.com/there's also a great review board on fm.com
also, one last thing to remember, the rebel, xt, 10D, 20D, and 30D all have a 1.6x crop...if you want a full frame camera like a normal slr, you're going to have to move up to the 5D or one of the more professional cameras.