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.