Which lens is better for low light concert photos?

I have a Canon D40 and also a Canon EOS XSi I currently have a 24-85 3.5-4.5 USM zoom which I use all the time close up and it works great. I also have a Canon EF 75-300mm f/4-5.6 IS USM which I cant seem to get good low light photos with. I was researching and the concensus seem that I should get a Canon EF 70-200mm f/2.8L IS II USM and it should do well in low light I am willing to fork out the bucks if it will work as good as they say Please help?