Re: Help with Nightclub photography!
If you get to do the gig again, ditch the zoom and get a fast prime - it doesn't need to be super high quality F/0.95 or anything, just a compact equivalent of 28mm (in 35mm film terms), and a compact flashgun (don't have something like an SB900) with a diffuser - this means that you can carry it around all night, and when you do take a shot ther eis less chance of someoen bumping into a large flashgun and hearing that horrible crack as either the hotshoe or hotshoe adaptor gives up the ghost....
Cropping to give portraits etc when using a short focal length in this situation is perfectly acceptable, so don't sweat the megapixies/magnification here - it is all about capturing the moment.
Humans need Fantasy to be Human. To be the place where the Falling Angel meets the Rising Ape.....Terry Pratchett, Hogfather