You don't need to uninstall Office 2000. The Office 2007 installer will delete it automatically, though you can customize it to retain Office 2000 as well. If you do retain Office 2000, note that only one version of Outlook can be installed at a time - if you install Outlook 2007, Outlook 2000 will (AFAIK) be uninstalled and all emails etc will be migrated. For Outlook questions generally, see:
http://www.outlook-tips.net/