At long last, hard drive usage problem figured out.
http://discussions.apple.com/thread.jspa?threadID=2133932&tstart=0
Well, I don't know what all happened, but Safari 3 began to act up about a year ago, locking up periodically, and progressively got worse until this week when it froze so often that it was no longer usable. The freezing clears itself after 5-30 minutes or so, and until allowing it to "unfreeze", surely it would refreeze shortly after a force quit + relaunch. Firefox is too slow and hogs too much memory on this 400MHz beast but I decided there wasn't anything to lose by loading up Safari 4.1.3. Immediately, the thrashing returned. It has survived through a brand new hard drive with a fresh install of OS X 10.4.11. It must be a G3 thing.
BUT now I have a nifty app called "Little Snitch", and it showed that Safari made a short network connection to safebrowsing.clients.google.com, right when all the noise started. So I blocked this domain and turned off "warn when visiting a fraudulent website". Safari is now running perfectly.
So anyone googling for Safari excessive hard drive activity, maybe you'll find this post since I can't add to the original one.
PowerBook G3 Pismo 400 MHz, Mac OS X (10.4.11), 512 MB RAM, AirPort