What multitrack recording software are you running on the imac? Thats where I would start. I'm assumin the IMAC is only equiped with a standard sound card, if you are going to do just audio then thats fine for starters but if you wan to record also using midi which i recommend because of the numerous sounds available. If you go with using midi also the you will need either an external midi interface or a midi card(I'm not fimiliar with IMAC products, I use a PC). If you plan to run your sounds from the imac to mixer than you will need a multiple output audio card, where you can assign output channels from your recording software.