I agree with most of the comments here- a flash is going to be much more help grabbing indoor shots than a fast lens (assuming you're in situations where a flash can be used). You definitely want to get one with a swivel head so that you can bounce the flash and get more natural looking light.
On a crop, I found 50mm lenses to be way too long for indoor shots unless you're in a large space. I've shot events and my favorite lens on a crop was the Sigma 30 f/1.4. If you want a zoom, the 17-50 is great, especially for indoor events where you want to capture more of the scene. If you need more reach than that for concerts and outdoor competitions where you're not too far from the action, then the Sigma 24-70 f/2.8 (non-HSM) is within your price range and is also a great lens. 28 wasn't wide enough for me on a crop. The extra 4mm really makes a difference if you're in tight quarters. If you want a good lens for indoor competitions, you'd definitely have to get a prime if flash isn't allowed. Even f/2.8 generally isn't fast enough to capture fast motion indoors without flash unless it's really really well lit.