Within that budget, probably used 550D/50D and sigma 70-200 f/2.8 (the 550D has a newer better sensor with better high ISO performance, but the 50D has faster operation (faster max frames per second continuous shooting, better AF, more external controls)). You can get 550D's new pretty cheap at the moment since the 600D came out. Personally out of those two I'd take the 50D for it's faster operation, but the 550D will be more similar to the 450D you're used to (might be a good thing or might not).
The sigma 70-300 is f/4-5.6, which is really too slow (slow refering to it's small max aperture) to be used indoors. You won't be able to get a fast enough shutter speed, so you end up with motion blur and/or camera shake. Even with the 70-200 f/2.8, you'll probably be pushing the ISO up a lot indoors to get fast enough shutter speeds.
I'd wait for someone to advise you which version of the sigma 70-200 f/2.8 though, there have been quite a few different versions over the years, each with a slightly different spec and some seem to perform a lot better than others.