xlr8yourmac has posted a review and install guide to the G3 500 Bluechip upgrade card for older PowerBooks. I don't know if anyone still wants an upgrade after the release of the Titanium PowerBook G4, but it's up to you.
MacCentral has some news on how some guys at UCLA are developing software to help set multiple Macs up in bigger clusters, working in parallel. At the moment they seem to have sucessfully clustered 25 Macs. I guess with quite impressive speeds.