You can check out this link for details on print sizes, DPI & resolution:
I find the numbers a little conservative, but thats just a hunch and not based on any experiments I've done. You can also enlarge using software that will result in a much larger print with little or no artifacts. Since you also want good low light performance, I'd say spend some time looking at the 5d mkII as the low light is good (according to reviews I've read, I have neither of these cameras), and at 21mp, I can't imagine you wanting anymore. The only downside to the Canon might be continuous shooting speed, which might be handy during a live concert.
Just beware, full frame, large MP count comes at a price (Sony, Canon, Nikon, Whoever):
- You will need tons of storage. Both CF cards and hard drive.
- Your computer needs to have plenty of power unless you don't mind waiting 15 min. for a file to load in photoshop.
- Full frame lenses do cost more than "DT" only lenses.
- No built in flash on most of these cameras.
- Heavy as a mofo
, up to 7 lbs in some cases with a high quality lens.
If you don't mind, what sizes are you typically hoping to print?
If you've never owned a DSLR before, maybe you probably shouldn't start off at such a high level. Maybe a D300 or 50D or A700 might be enough? You can still buy quality FF lenses, and upgrade after a couple of years, but not have to spend so much on a body until you know you really need that much (and can use cameras that tend to be a little more complicated)?
Also, maybe try renting for 1 week. See if you really need a 3000.00 camera.
You could also check here for some moral guidance
(this is my struggle, not sure if it applies to you or not) http://luminous-landscape.com/essays/want-need-afford.shtml