FWIW, I'm running 10.5.8 on my G4 Cube, with a 1.7 Ghz upgrade from OWC, 1.5 gig ram and a Radeon 7500. It's as snappy as my C2D MAcbook Pro late 2008 on everyday tasks, but cannot be trusted for HD vid and such, but to ageing 100Mhz bus.

But I still love my "PowerCube" to bits!

Cheers