IMO, there won't be much difference in the low light capability between sub-$1000 bodies; they will all do pretty well up to ISO 3200 or perhaps even 6400. The other considerations you should think about for the body are the brightness of the viewfinder, the ease and speed of adjusting controls, and frame rate. Right now, I'd say the best buy would be a refurbished 50D body for $703.36 direct from Canon. It has a pentaprism viewfinder, more direct control of critical controls, and shoots at over 6 fps. Another great choice would be the Nikon D90.
A bright lens for low light will be more critical and potentially more expensive than the body; a 70-200mm f/2.8 lens is $1000-2500. Prime lenses are cheaper and faster, but not as versatile.
I need to take photos (and occasionally video) of professional dance and theatre work. The lighting conditions are frequently quite low, so I need a camera that can take action shots (dance is basically sport photography, in highly variable/low lighting) in these conditions without artifacts. Photos are the primary consideration, although I'm attracted to the lenses on dSLRs for a wider range of video work than I'm able to do on a dedicated video camera, so the video function of this dSLR is also a big consideration. Due to friends' recommendations, I'm considering the Canon 60D or the (not-yet-released) Nikon D5100, but would be open to any other suggestions. I'm a relative newbie with dSLRs, so a Nikon D7000 or a higher end Canon seems like overkill -- I need professional quality photos, but a fairly basic pro-sumer camera that can handle low-light, highly dynamic conditions. (I run a professional theatre company, and have been let down by others' photos in the past, so I prefer to be self-reliant -- I'll have a learning curve when I get this camera.) I know I have to pay close to $1K to get a camera (not counting lenses) to do the kind of work I'm looking for and would like to keep the price for the body under 1K so I have some budget left over for lenses. Thank you in advance for any advice or suggestions you may have.