Been there done that and got the T shirt for this sort of thing. I shoot now with a D50 and D80 to broaden the focal range I can cover. I sometimes use 3 cams!
The basics:
1. Shoot RAW and PP
2. Use the Incandescent WB preset. (Mostly you cannot set a WB because the lighting colour changes all the time.)
3. You are likely to need circa ISO 1600 f/2.8 and 1/60th. So you will need f/2.8 constant zooms or primes. You can see what lenses I have in my Profile. (Lighting varies enormously from stage to stage and set to set)
4. Often with a black set you will underexpose at ISO1600-f/2.8-1/60th. You will have to push process your shots in that situation.
5. Set single area focus. Turn off the focus assist beam (and the focus beep!).
6. As for metering there is no easy solution. In a church with constant lighting I shoot Manual taking test shots but when photographing a stage dress rehearsal the lighting constantly changes so that is not possible and, usually, theatre time is money so there is just one real onstage rehearsal so you have no chance for test shots on an earlier occasion. I use Matrix Metering. It usually gets it nearly right and I can PP the rest. In long scenes I can check for Spot readings and apply some exposure compensation if appropriate. Sometimes I just know it is wrong and overide it (but shooting two cams you have your hands full anyway).
7. Set you cam(s) up to get good closeup shots and then try your best to get wide angle for context shots as well. Shots which show the whole of the stage with miniature figures on it quickly get boring. You want to capture facial expressions (and sometimes in strong shadows).
8. Try to get a few angled shots on a monopod for variety. (I like to use my 50mm f/1.4 for these shots)
9. Shoot lots and be highly selective as to which to PP (I typically shoot 400-500 shots but try not to exceed 50 shots online for each production).
10. Be prepared to spend lots of time doing PP. You may have one second to get the shot. If need be you have hours to get the best out of it.
11. Take look at my recent galleries they may help a little (but do bare in mind a) that I am shooting RAW and the recorded exif may be a stop or more way from where I end up with the finished product and b) I am sometimes on stage and therefore reliant on others to take shots at that time (another reason why I stick with MM). So if the exposure settings are odd I was probably thinking that too!)
12. You may want to think now about how best to present your photos to the theatre group. I used to produce CDs for everybody but that got time consuming. A photo gallery is better.
Constant f/2.8 zooms rarely exceed 3x zoom. This is a bit limiting. If you can borrow a 2nd Nikon body that would be very useful. You can then keep lenses mounted and do the minimum of changing in the dark. If you are able to go down that line you can mount both cams on a T bar on your tripod (Fairly cheap to acquire via Ebay) or use the 2nd cam on a monopod (Try asking the theatre group. Someone may have accesss to a say a D50. That is how I got access to my 3rd body!). If you do this it is essential to synchronise the clocks on all cams!!!
If there is more than one dress rehearsal you can shoot different lenses for each rehearsal (Oh luxury!!).
Lenses:
cheap and cheerful (but effective):
Nikon 50mm f/1.8 (or better f/1.4)
Nikon 85mm f/1.8
better:
a WA f/2.8 (I use my Sigma 17-70 f/2.8-4.6 at 17mm f/2.8) as if a prime. There are no cheap and good primes that wide)
Tamron 28-75 f/2.8
Sigma 24-70 f/2.8
Nikkor 35-70 f/2.8
best:
Nikkor 80-200 f/2.8 (mine is a D ED two ring with tripod mount)
Nikkor 70-200 f/2.8 VR
Hope that helps. Do come back if you have any specific questions.
These galleries should give you some ideas:-
http://PlacidoD.zenfolio.com/p16885626
http://PlacidoD.zenfolio.com/p1045531707
http://PlacidoD.zenfolio.com/p43741400
But by all means look at them all,. hopefully they show some sort of learning curve over the years!!
--
Chris Elliott
Nikon D Eighty + Fifty - Other equipment in Profile
http://PlacidoD.Zenfolio.com/ 
