This is an old procedure, which is commonly understood by PostgreSQL administrators. However, if you’ve not done it before, over at the FreeBSD Diary there is a nice article describing how to upgrade the client and the server when moving from one version to the next (such as from 8.3.x to 8.4.x).
The basic thing to remember is that a complete dump of the databases is necessary, with the appropriate reload process. The article describes this process well.
Anybody who is interested in FreeBSD should follow the FreeBSD Diary; this “blog” has been around longer than blogs have been.