Google Calendar does not support attachments. If you wish to keep your calendar attachments, you must save them to your computer or your H: drive. Once the calendar copy is complete, you will receive a report of all calendar events that contain attachments so that you can easily locate and save them. However, if you use Outlook you can start identifying, reviewing, and saving your calendar attachments now.
ArgoMail and Entourage: Unfortunately there is no easy way to do this in advance using ArgoMail or Entourage. Your best bet is to wait and use the report you will receive when the calendar copy is complete.