I have used a Nikon D3 for the past eight years and its low light abilities are brilliant. (Can't wait to get my hands on the D5 which on paper appears to move the bar up a couple of steps or so).
However, I often still shoot gigs at much lower than ISO 6400, as it does create 'better' pictures, athough noise on gig pics is less critical than say a wedding or formal portrait. Bear in mind lighting can also be very bright too. But with anything less, in a small venue where the lighting is non existent, I wouldnt bother.
Strangely, of all my lenses, I like the f2.8 70-200mm which allows shooting from the back of the venue to incorporate some measure of the crowd, or for very tight close ups of performers faces etc if shooting from the side of the stage or mosh pit.
Of course with that sort of lens you need a pass in most cases to get in, unless it is a very very small venue, in which case I might revert to my f1.4 50mm.
Of course, with gigs, you are never quite sure until the band are on stage as to how the lighting will be set up, or how large or small the crowd are. I find being at the sound check often gets some great shots as the band are more relaxed (or stressed on some occasions!) and the lighting is usually brighter. Mixed with shots of the actual gig helps create a good portfolio.