It really depends on what you want the pictures to look like. I recently shot the last two groups of an outdoor festival and used two bodies: A 1.6x crop body with a 70-200mm f/2.8 lens and a full frame body with a 24-105mm f/4 lens. I chose the 24-105mm lens over my 24-70mm f/2.8 lens due to image stabilization on the 24-105mm f/4. I had stage access and the stage was a temporary outdoor one that was less than rock solid in terms of stability. The intensity of the theatrical lighting was highly variable. The spotlights on the lead singers was much brighter than the rest of the stage lighting.
Here are a couple of images taken with the 24-105mm lens (both images are the same resolution, but this page apparently re-sizes based on width, rather than maximum dimension):
24mm, ISO 3200, 1/50 sec @ f/4 from rear side stage
55mm, ISO 2000, 1/320 sec @ f/4 from the media pit
Here are a couple of images taken with the 70-200mm on the crop body:
200mm, ISO 3200, 1/1600 sec @ f/2.8 from front side stage
70mm, ISO 3200, 1/320 sec @ f/2.8 from side stage
Theatrical/concert photography is one of the most challenging forms of photography there is, both in terms of pushing the equipment you use to the absolute edge of their capabilities and in terms of requiring every bit of skill and experience you might have as the photographer.
To do concert photography well (and get consistently good photos instead of only the rare keeper) you really need premium lenses more than just about any form of photography other than shooting birds in flight from a distance. So while a "step up" lens would be better than a typical kit lens, you really need to go for lenses with the widest aperture and fastest focusing that you can afford. And in low light the roughly one stop advantage a full frame sensor gives you over an APS-C sensor is also often the difference between getting the shot or not.
Photography is the art of capturing light. Most concerts don't offer
much light to capture and what light there is to capture is changing
rapidly and the subjects are usually very animated. So the traditional
solution to not much light (longer shutter speed using a tripod to
hold the camera still) doesn't work because nobody on stage stands
still for 10-15 seconds while you take a picture. The traditional
solution to capturing motion (faster shutter speeds) doesn't usually
work because there isn't enough light to capture a good image on a
small sensor with a narrow aperture. In the end you have to balance
the two as best you can AND use gear that allows you to capture as
much of the scarce light that is present in the scene in as fast a
time as possible. That means fast lenses (wide apertures), larger
sensors, and cameras that are highly responsive (fast handling).
So zoom lenses need to be f/2.8. Prime lenses in the normal range (e.g 24-80mm) need to be f/2 or wider. Even at those apertures you will often find you must shoot at ISO speeds in the 3200-6400 range to be able to stop the motion on stage.