I was hoping to stay at 400 to 800 iso, but I know I will have to keep my shutter speed at in the mid 100's.
You'll need faster shutter speeds than that if you want non-blurry shots of moving dancers without a flash unit to freeze the motion.
Does the RebelXT have the "Safety shift" custom function? If it does you can set your shutter to something like 1/400 in Tv mode. Then if the light isn't good enough for your widest aperture, it will slow down the shutter to get the exposure right.
Don't be afraid of high ISO settings. Digital noise can be dealt with, but no amount of post-processing can save a blurry picture.

