Hi, I have a new C2D iMac 17" 2 GHZ. I am interested in getting new external speakers. I was wondering, since this is my first computer with optical audio out, if there are certain speakers that take advantage of this, what the benefits there are, what to look for. Is optical audio primarily for adding 5.1 sound? Is the only app that produces 5.1 sound the DVD player? Thanks!