> my MacMini 1.47MHz

It's either 1.42 or 1.5 GHz (unless weirdly overclocked from 1.25 or 1.33 GHz).

> On a PowerMac G5 (A1047) I understand the OS has a 1.5G memory limit

The MorphOS memory limit applies to all compatible machines. And koszer has it (almost, hehe) correct: it's 1.7 GiB (= 1.8 GB).