Just a quick note of instructions for Leopard.
1) Backup Address Book File > Export > Address Book Archive … Agree to the default file name, saving it with a date
2) Backup iCal File > Backup iCal Agree to the default file name, saving it with a date
3) Quit all applications on the iMac. Then open iSync in Applications. iSync menu > Reset Sync Data Reboot
4) Let syncing happen. If it comes up with conflicts, review them, and choose the item in each conflict most likely to be accurate.