Before upgrading the software make a copy of your PST file. You can search for it to find it. It’ll be probably be fairly large.
The upgrade should go just fine by itself. It will read the settings from your current Outlook 2000 and import the current PST file into Outlook 2007. Just in case there is a problem you can use the copy of the PST file. Just point Outlook 2007 to the backup of the file and use it as your default file instead of the original file. The format of the PST file has not changed between Outlook 2000 and 2007 so it should work just fine.