Thanks so much for that. I've had a 350D since its early days so I'm jumping into vast new territory with all this. Cheers for the great answer.
EDIT: Man, so I just checked out the ST-E3-RT Transmitter..... No autofocus assist beam!? What good is it then, I see myself doing Wedding Photography & the like in the near future, so this would be practically essential... I'm starting to think I'm better off just picking up another 600EX, Which is probably what Canon really wants... I call Shenanigans Canon!!!
I picked up a second 600EX-RT simply for the fact that my master control can also flash. Why not? If you have the ST-E3-RT Transmitter, sure it'll do the job, but it can't flash. With wedding photography it's nice to have an on-camera flash and an off-camera flash somewhere else. This way you have it all, because if you don't want the on-camera to flash, just tell it not to.