If you are just starting out, I strongly recommend multiboxing three mages. Since you've played them before, like I did when I started, you will have a super easy time constructing macro's that work well with your MB'ing play style. I personally found Warlocks to be the second easiest, and Hunters the third. The mage class is probably the easiest class to use to get the feeling of multiboxing before trying other classes, but thats only imo.