If 4 or 5 copies is all you want you will need a large tower, one of those server like towers. If you plan to use one of the burners as a reader, and then burn the information on the others from an image created, then you will not need an optical drive dedicated to be a reader. I think personally having an optical as a reader is a better idea. Also, although it isn't necessary, IMO its a good idea to have all exact same burner optical drives. Depending what you plan to make multiple copies of, data or music, get any current 52x ASUS, Lite-on, or Plextor. I would personally go with the Plextor if this will be meant for audio. If you ever plan to use this setup to backup a protected game, get ASUS or Lite-on. Next you must consider SCSI or IDE. In either case you will need add-on PCI cards (possibly more for IDE) to handle all these drives. make sur eyou have a good power supply, cause these will be a greater drain than an avg PC, go with Antec True Power line or Enermax, or Thermaltake. Also get a good motherbaord, I have personally always been an ASUS fan, and get some nice dual channel DDR 256 or 512mb depending on your spending limitation. Any CPU over 2ghz will easily handle these tasks, and you will need software that can burn on multiple opticals, I think DiscJuggler is best at this and they started the entire multiple burning deal anyway.
Any other questions or thought?