You can just copy the PST file of Outlook 2000 and Open it in Outlook 2003 and you will get all the emails there in Outlook 2003. But still the PST remains in the format of Outlook 2000 - ANSI format, which has got the file size limit of 2 GB. So if in future once your PST grows above 2 GB it will get corrupt. So you need to upgrade the Outlook 2000 PST file to Outlook 2003 - Unicode file format having a limit of 20 GB.

Get the PSTUPGRADE tool to quickly upgrade older Outlook 2000 PST files to Outlook 2003 files having bigger size limit. Download the software from http://www.pstupgrade.com