Exchange 5.5 to Exchange 2000 migration

Were running Active Directory, w/ a 6gig IS store db on Exchange 5.5 but were running out of space and would like to migrate to Exchange 2000 on a win Adv. server or Srver.

What is the best way w/ less downtime to migrate mailboxes/connectors etc.. to the new server w/ less downtime for users. Any step by steps ??
thanks