I have a Canon 550D, stock lens and also a 70-300mm zoom. I often find myself taking photographs in very low light but where there is movement e.g. a pop concert, and I'm always disappointed that my pictures are grainy and not as well focussed as i would like (I was shooting with the kit lens at ISO6400 and 1/100 at the weekend and not really getting away with it).
So my question is: what lens, or sort of lens, should I be looking for to maximise the light I capture? I don't need a huge zoom, just the ability to capture a crisp image at low light.