well first thing to decide is Canon or Nikon
I prefer Canon but thats just me
your standard basic low light lense is the 50mm F1.8 (approx $50) - best value low light lense there is
so basically the lense is 50mm (fixed lense with no zoom) and the F stop is 1.8
you can go down to 1.2 and 1.4 but they are generally very pricey
you need to be relatively close to the stage with this lense though
another good lense for concert shooting from the tapers section at most shows is the
85mm f1.8 (approx $300) - also a fixed lense but you can be a little further back from the stage to FOB and your going to get good results.. this is also an excellent general use lense for the price range
as for the camera body - my advice is to just get that
dont buy a kit.. as the lense that comes with the kit is usually a waste of time
so basically your gonna be spending about 400-550 for the body
I run the Rebel XT which is 399 (8 megapixel) and I am very happy with it
i get all my gear at B&H here in NYC..
i can give you links for all the stuff if you want too..
one other thing you can do is go on a site like flickr.com and you can search for the camera and the lense and see picturs that people have taken with those setups...