Use Migration Assistant to move everything over to the new Mac. If you do a back up on the PPC before you use Migration Assistant, it should transfer over to the iMac, along with all your programs, settings, etc. Then a sync should go no problems, but just to be sure, make sure everything is in place on the new Mac (Addresses, etc), and tell DM to overwrite everything on the phone to give you a clean slate (from a sync perspective), since BBDTM for Mac uses Sync Services. This should prevent any duplicates from being created.
All that is necessary for the triumph of evil is that good men do nothing
Edmund Burke 1729-1797