*Place the camera in the manual mode with your shutter speed set to the highest flash sync speed (1/200th second) it supports.
*Put the SB600 in its "TTL" mode
*Take one or two test shots to refine your aperture setting.
Done
Also try shooting without the flash with your cameras white balance set to incandescent and ISO 1600 with the lens wide open in the aperture priority mode. These shots, while noisy, will catch the lighting of the stage and band members