Straight from the horse's mouth.

Mac OS 8 and 9 compatibility with Macintosh computers

This page documents which Macs (from the Quadra and Centris lines right through to the G5) will run which versions of Mac OS 8/9.

Most, if not all, PCI class Macs will run OS 9.2.1 and 9.2.2 with the assistance of OS 9 Helper.

Peace,
Drew
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Power to the PowerPC!