The money saved will probably be used to get scraps of food of off garbage men.
The 17-70 is also cheaper than the 17-50. ($450ish compared to $650) :/
I'm going to be shooting indoors mostly with variable lighting. (Really low to ok to bright) Mostly dancers, I imagine.
The 17-50 Tammy runs for $400ish new. You might be looking at the VC (Tammy's version of IS) version which isn't really worth it.
Trust me, this 17-50 lens is solid. The Sigma 17-70 isn't as sharp.
Honestly, just stick with the kit lens IMO and get a flash. That's been 10x as useful for me than having f/2.8 capabilities.
There honestly aren't that many applications for f/2.8. You can still get nice bokeh shooting f/4 which I shot for my last vacation. And if you do group shots, f/2.8 risks blurring other people.
If you want a fast lens, just grab the 50mm/1.8 and learn from there. I personally have a f/2.8 lineup (11-16 and 17-55), and hope to expand that to telephoto someday....
Stick to a T2i or better. I have a T1i, and while it's great, the 7D sensor which they use in the T2i is amazing. I shoot both a 7D and T1i. I won't say it's world's better, but it's definitely a step up, and a T2i shouldn't be that much more than a T1i. All I say is avoid the XSi if you can. While that sensor is great, the 230k pixel display is crap compared to a 920k display. So T1i and up at least....
BTW, regarding the kit lens, it's just as sharp as top notch lenses like the Tammy 17-50, or the $1100 Canon 17-55 f/2.8 lens. The Sigma doesn't even come close at all. The Canon kit lens is top notch and if you don't desperately need low light capabilities, I suggest you learn from there. I could've shot my friend's wedding with my kit lens if I wanted. Most of my 17-55 shooting was done with flash, and when I didn't use flash, it was with the 70-200.