Using Quartz Extreme on an old system is a mixed bag.. it speeds up stuff under light load but saturates the (66MHz, in your case) bus and slows things down with heavier use.

I'd call 10.2 "tolerable" on my beige 266 if you've got reasonable expectations for a 5 year old system. Mine mostly just gets used as a file server/place to connect HDs nowadays though. As Drakino said, stuff all the RAM you can in it.. if it's a desktop, take that literally; you can coerce not-quite-low-profile DIMMs to fit in there if needed.