I'm trying to work out which DSLR to buy. One of my main use cases is photographing live bands in dimly lit bars. This often involves fast movement (particularly drummers) as well as low light. Which features should I prioritise in a camera for shooting in this environment?
The most important body features are:
The max ISO levels (and the noise levels at high ISO)
Low light shooting is much easier at high ISO settings, but many lower end cameras have trouble with noise as you increase the iso. A good indication of the high ISO performance can be found at www.dxomark.com by looking at their "Sports (ISO)" rating for the camera.
The camera's low light AF performance.
Some cameras simply do not handle autofocus in low light as well as others.
The camera's continuous shooting fps.
To get the right shot, many use continuous shooting to take a series of shots with the idea that at least one in the series will be sharp.
Moving away from the body, also keep in mind that a fast lens (nifty fifty or similar) will help both the exposure and the camera's ability to auto focus. Also, In some situations you may want to use a flash. An off-camera flashgun is a good candidate to avoid the direct glare on the cymbals/guitars, but this is often not possible since it distracts from the show.
The single biggest feature will be strong high ISO performance. There are a number of cameras that fit that, much will depend on your budget. In general, the full-frame dSLR options will perform very well at high ISO, but in the more budget realm, the current king of the heap seems to be the Pentax K-x. After that, you need fast lenses, the wider the aperture the better. So, things like a 50mm f/1.4 and the like.
That's just the basics for low light, but I think there are some concert shooters on the site that can give you some good tips on that as well.
Are you going to be close to the band - like on stage or behind the stage or very near?
I am assuming you are close to the band: For drummers, you typically want to use moderate shutter speed instead of very fast, since you still want the hand movement to be blurry (from movement) but yet most of the person to be quite still. Use wider lens to get the whole drum kit + drummer. If you elect to use flash, you can use the rear curtain.
If you are far from the stage: What @chills42 says and a tele-photo lens.
In addition to all of these things, make sure that your camera can autofocus with fast primes. For instance! For the longest time, Nikon D40 users were left out in the cold when it came to fast primes; now, they have the 35mm and the 50mm, but still can't use the 85mm (at least, in the Nikon lineups). When I've shot bands, it's been with these fast primes rather than zooms, and I found manually focusing to be very hit-and-miss.
As has already been said, low light performance is the most important thing from a body to shoot live bands.
I love my Nikon 50mm / f1.4, but for live performances, I find that you are often restricted in your movements (most venues will not allow you to be on stage during a show). So I tend to prefer to use a zoom. I also like close-ups, so most of the time I shoot with a Sigma 70-200mm / f2.8.
As an aside, keep in mind that the public is there to enjoy the show, not to enjoy the photographs. I find it very annoying when the first rank at a concert is busy with 6 photographers who dont care if they ruin the show for everybody else ...
In my experience a fast lens is the most important thing for shooting in low light environments. As other posters have pointed out, high ISO is helpful, but most Canon or Nikon DSLRs now have sufficiently decent low light performance. The biggest determinant is going to be the lens, where you really should get something good. A versatile 70-200/f2.8 is a good one, with a 24-70/f2.8 (or equivalent) for closer up. I've had lots of success even with older Nikon D70s and D100s as long as I used a fast lens.
Even with fast primes, you may need a small LED flash light. If you go this route, you will have to set your exposure and auto-focus separately, but most cameras allow this (and many people prefer it!)
I think there are a few flash/body combos out there that will allow you to use the auto-focus assist light, but not fire the flash, but they are tricky to figure out. I believe the Canon ST-E2 will also provide this function for you.
You could of course learn to manual focus, but this can get tricky if you are trying to work fast.