You should go to list view in your calendar, Go to menu bar "Edit > Select" All to highlight all records. Then choose menu bar "Assign Tags" to assign the Sync tag. Then you can do a sync. Events and tasks with the Sync tag would be the only ones that get synced to iCal.

For integrity's sake, I launched BusyCal and clicked on List view selected the popup menu to show Decade (default is Month). Then I sorted by date and double-checked for duplicate events.....

Apple's sync services have been wonky with a variety of programs.

When I tried out SOHO Organizer and Contactizer Pro, contacts seemed to transfer well (minus the links, attachments, and custom date). But calendar info seemed to be blitzed.

SOHO Organizer only captured the last 3 months from my iCal calendar.

Contactizer tried to sync and made duplicates of my calendar.

Before doing a sync, I would do a SuperDuper clone of your hard drive so that you can just go back.

BusyCal seemed to just see all my data. It's my calendar of choice. It's what I wished Now X would become. Hopefully a contact solution will be coming soon. I'm also hoping that iPhone OS 4.0 will finally show a calendar app that syncs tasks. But I can work around that because I am currently using OmniFocus for iPhone as my task manager.