I've found for low-light, you need a prime, and a lens generally faster than f/2.8. The EF 35mm f/2 works pretty well, but it is a tad bit slow in focusing, and can hunt sometimes. A zoom does offer flexibility yes, but I think you will find it not quite fast enough, unless you run ISO > 3200. The disadvantage of a prime can be somewhat overcome you can move around wrt your subject in low-light.
I use the 35mm focal length on a 7D at ISO 3200-6400 to photograph belly dancers in extremely dim and contrasty club lighting. The 7D has a FoVCF of 1.6x, so this focal length works out to be 56mm equivalent view. I'd say at a working distance of ~3m, with this view, the belly dancer fills the frame with some edge room for veil flips above/around her.
If you are willing to up the ISO, and you have dim, but not extremely dim light, the f/2.8 zoom can work well, and of course would give you more compositional freedom. That particular lens though, I don't know how sharp it is wide open. It could be you need to work at f/3.5-f/4 to get an acceptable sharpness, and that of course militates against low-light use.
As a related matter, in case you'd not heard of this technique, you can definitely improve your autofocus in dim light by using selective autofocus, a feature of the camera where you place the focus dot (or zone, an option for the 7D) on the area of interest. This is *much* better than using full autofocus, esp. when dealing with moving people. This way, you don't get the background in sharp focus, but the person you were photographing is blurred.
Just another comment ... the lens performance is fairly unrelated to the ISO performance of your camera. That is to say, the lens itself will not fare worse, if the ISO is increased.
Hope these are helpful comments ...
