Hello Heather, great to hear from you! I was wondering how you were getting on with your XTi, so it's good to hear it's working out!
It's also great to hear you've identified the kind of shots you want to take and shortlisted some quality products.
Here's my take on each of the lenses you've mentioned:
1: Canon 17-40mm L - this is a great lens, but at f4, it doesn't really offer a significant brightness upgrade from your kit lens; it's only one stop brighter when zoomed-in, and actually a fraction darker when zoomed-out. Sure the focusing's faster and the quality better, but it has a shorter range than your existing kit lens, so with this and the aperture, I don't think it offers sufficiently compelling reasons for you to upgrade for the reasons you want it.
2: Tamron 17-50mm f2.8 - I've not tested this lens, but I've heard good reports on the quality. It's certainly nice and bright at f2.8, but I'd check the focusing speed, as it may not be as quick as the Canon USM motors in the other three lenses you're considering.
3: Canon 10-22mm - as you say, this lens is great fun, and you'd be able to get some really fun shots of big cars and your kids playing around. Just zoom it out and get right up close to your subject. It's great quality too.
4: Canon 17-55mm f2.8 - it may be pricey, but it sure is a serious piece of kit. The f2.8 aperture is nice and bright, the focusing is quick and about as good as you'll get from a zoom under low light, and the IS really is useful even at this focal range. I'd say it's the ultimate Canon zoom for EF-S body owners who want to work under low light.
So in an ideal world, I'd say buy the Canon 17-55mm f2.8 for general use and the 10-22mm for extreme fun shots if it sounds appealing to you. Or if not both, then just the 17-55mm f2.8 for now.
Here's another thought: if you loved the brigtness of the 50mm f1.8, but found it a bit too long, you might want to consider a nice, bright wide angle lens?
I'm not suggesting the 24mm f1.4 or 35mm f1.4 L models - unless you win the lottery! - but the Canon 24mm f2.8 and 35mm f2.0 are both quite affordable and should focus at around the same speed as your 50mm f1.8. You will find the USM models focusing faster though, so the Canon zooms we've mentioned have that advantage.
You did say your main lens has to be good indoors in low light though, so to me that says go for the Canon 17-55mm f2.8. You'll love it!
It is quite big and heavy though, so I'd recommend trying it in person first if possible - it'll be quite a shock compared to the 18-55mm kit lens or your 50mm! If it is too big and heavy, but you still want great low light performance, then do think about the fixed focal length lenses like the 24mm f2.8.
I'm sure you've already seen it, but we have a group test of Canon XTi kit lens upgrades including the 17-55mm f2.8 which you can find here:
http://www.cameralabs.com/features/Cano ... s_upgrade/
We also have a review of the Canon 10-22mm here:
If you decide to go for either the Canon 17-55mm f2.8
or Canon 10-22mm
(or both!) and were thinking of shopping through Amazon, please consider clicking the lens names here to enter their store. The price is the same, but they give us a small commssion which helps keep Cameralabs going!
Actually, you might want to click through to Amazon's 17-55mm page for a quick look anyway as it has some interesting owner opinions on it, several of which are compared to the 17-40mm.
Let us know how you get on anyway and shout if you have any more Qs.
In the meantime, what does everyone else think? Should Heather get the 17-55mm f2.8?