Hmm.. I am currently trying out MorphOs on the basic Mac Mini G4 (1.25Ghz), I like the small form factor and the quietness of the machine, but I do suspect that eventually I will struggle with it. I guess upgrading to the 1.5Ghz one would be the minimum.

My biggest issue with Power Macs is noise, the liquid cooling ones are also a big no, and they don't really look as nice as the Minis.