Afaik MacOS X didn't boot on old world PPC machines with NuBus slots, but 10.2.x can be installed on a PowerMac 9500 with a G3 upgrade and with a 32MB PCI Radeo

Afaik MacOS X didn't boot on old world PPC machines with NuBus slots, but 10.2.x can be installed on a PowerMac 9500 with a G3 upgrade and with a 32MB PCI Radeon GPU the UI was even usable. It helped that the box was also upgraded with >800MB RAM spread over 12 DIMMs as well as two 10kRPM U160 disks in HW RAID1. Still the slow 50-60MHz PPC60x bus was beyond painful.