Hard to go wrong with modern dSLRs these days. As mentioned the lens will probably make the biggest difference if you are buying a new body with good high ISO support.
I have a Pentax K-5 that I got for about $1000 (body only) which leaves you about $1000 to spend on a nice lens or two. Like all the other new cameras the K-5 has good low light performance that gets better with a fast lens.
If you want to be inconspicuous, the K-5 is one of the smallest pro (ish) bodies. It's also weather sealed which can come in handy if you like shooting outdoors too.
For your needs, I'd look at the Pentax 50-135 2.8 but that kills the budget by about $500.
For less money the Sigma 50-200 2.8 or Tamron 70-200 2.8 are also both considered to be high performers and would also fit your budget.
I'm assuming you would want the reach these longer lenses provide for concerts, but if shorter works well enough these same manufacturers have similar wider options too.
I shoot more with wider lenses and the concerts I'm likely to shoot are in smaller clubs so a Tamron 28-75 2.8 is long enough for most concert shooting I'm likely to do.
For the darkest situations I have a vintage manual focus fast fifty (Pentax A 50mm 1.7) that I picked up for next to nothing on eBay. That lens is a great value!

Good luck and happy shopping!