In updating Postgres and plugins to v8.4, including a 50GiB database on an aging G4, I learned a couple things.

• Years ago I had used some incomplete info to decide that all PowerPC chips are 64bit by nature. This was partially true, and I'm still unsure about some of the details. Only the G5 is fully 64bit in that it can run 64bit programs on Leopard (though I don't have a spare G5 to actually test that).
• Postgres 8.3 does not work on PPC Leopard (32bit). I made the mistake of deciding to upgrade from Tiger to Leopard before upgrading Postgres to 8.4 (which also somehow corrupted the transaction tables which I had to figure out how to zap), then had to find an extra disk I could boot from Tiger to export my data. Postgres 8.4 does work on Leopard, so that leads me to believe that it's a bug that was fixed and not a build problem.