i have a 4gb nano and have its songs and playlists on my iMac. i just got the 8gb ipod touch which can hold more music/video than the nano. how do i set up separate "accounts" on my imac so that i can sync both the nano andthe ipod? And have another "account" with my son's music. a tech support persons told me to set up one playlist for the nano and another for the ipod--but then i do not see how i cld keep my current playlists for classical/exercise, etc.