The Sigma 70-200 f/2.8 runs ~$800-900 and is a fine lens. If you're on a tight budget and do not need IS, it's certainly worth considering. But chances are you'll still want some sort of IS when you shoot indoors (unless your subject is very brightly lit) though maybe a monopod would be adequate depending on you. Still, it wouldn't help you too much to trade lens aperture/quality for IS (eg. 70-200 f/2.8 vs 70-300 f/4-5.6 IS). One thing though about a 70-200 f/2.8 lens: it's a rather long, hefty lens at 7-8" long (w/out hood) and 3-3.5lb, especially if you plan to attach the XTi to it (not the other way around
). IIRC, the Canon version of that lens is the biggest/heftiest amongst the various brands too.
Anyway, if you don't mind going the used route, you might save some $$$ that way. For instance, you could probably easily get a non-macro Sigma 70-200 f/2.8 for <=$500.
I would probably not bother w/ the Sigma 17-70 f/2.8-4.5 (unless you get a good deal on a used lens perhaps), if I were you. Most 3rd party lenses just don't hold their value quite that well at resale, so if you're gonna buy something better than the kit lens, I'd recommend getting something that you know you'll use for the long haul, not some intermediate lens that might only last you for a year or two.
Also, do give strong consideration to primes, especially for the "normal" range from near-wide to near-tele since you want to shoot indoors. While the idea of shooting w/ fixed focal length lenses may not sound appealing at first thought, it's really not quite as inconvenient as one might think in actual practice. Many people don't actually use all that many different focal lengths on their zoom lenses. Often, they find they only use the extreme ends for the most part, and the occasional use of the intermediate focal lengths could often be substitute w/ a couple steps of the feet and/or somewhat different POV. And the restriction of using a prime can often free yourself in other ways, eg. you can be more quick and decisive, you're more likely to learn to see what works and what won't for a given focal length, distance, POV, etc., you'll have a brighter lens to work w/, etc.
As Scott pointed out, you can get the Canon 50 f/1.8 very cheaply. The Canon 35 f/2 is quite affordable too at <$250 -- I use a Nikon 35 f/2 a lot myself and find the focal length quite versatile as my "normal" lens. Many others prefer 28mm over 35mm for a "normal" prime, and Canon offers a 28 f/1.8 that costs ~$400. I'm not a fan of the 50 f/1.8 myself as I find it usually either too long for indoors or too short for most other occasions. Instead, I pair my 35 f/2 w/ an 85 f/1.8 most of the time. The Canon 85 f/1.8 goes for <=$350.
If you go for the kit lens as your first lens, you may indeed end up finding that incrementally adding a couple of primes to complement the kit lens to be a good way to go...
_Man_