I'm very comfortable shooting at 50mm on my 40D, so I ended up with a 50 f1.4. There's not much said about the Sigma 20 and 28 f1.8's, but the 30 f1.4 is very popular, and was another lense I was debating, but I think I want the Tokina 12-24 f4.
Review the pics you currently take and figure out what's the most used focal length. If you're shooting alot at 17-2x, but cropping when you get home, get a 30/50 prime and just compose better. If you're not cropping your pics, I think you should pick up the 17-55 f2.8 IS. f2.8 isn't enough for low light no flash IMO, but it'll replace your kit lense completely and not just for 1 aspect of photography (indoor shots).
But in the end, I'd say get a flash. I used a 580 EX with the kit lense for a bit and it works remarkably well.