The basics that you need to deal with are:
Full code review
Full regreshion test
Full backups taken prior to upgrade after all users are off the system.
Then do an uninstall and reinstall or an upgrade.
Followed by a smoke test prior to release to production.
Upgrading from SQL 2000 to SQL 2005 is a major upgrade and should be fully planned out before attempting to do the upgrade.
Are you going to upgrade on the existing hardware, or are you going to upgrade the hardware?