Upgrading from PostgreSQL 7.4.3

I'm running PostgreSQL version 7.4.3 on a Dell PowerEdge SC400 running FreeBSD 4.10. Fresh SCSI drives were installed in the server a week ago. A variety of Perl, PHP, shell and perhaps other scripts read read from the database, write records to the database, and otherwise shuffle data here, there and everywhere all day, every weekday. Rebuilding the server from ground zero isn't a viable option. I'm clueless what is dependent on anything else. Because the system was built by a number of people over a period of many years, there is no one person who understands the system in its entirety . . . other than when something doesn't work it's a headache.

Where do I begin the upgrade process? What litmus test can I use to separate those who have the requisite skill to do the job and those who don't? For someone with the necessary skills, what's a reasonable range of hours required for them to do what needs to be done?

Thank you in advance for your insights and guidance.