> if the G4 in the MacMini was available up to a 1.7GHz speed, I wonder if it would be
> possible to safely overclock the 1.5 chip to 1.7 or higher?

Check my link to Applefritter where someone stated he successfully overclocked his 1.5 GHz rated 7447B to 1.83 GHz. But that's always to be taken with a grain of salt because the 1.5 GHz Mac mini's 7447B is probably rated at 1.5 GHz for a technical reason. The existence of higher rated 7447Bs doesn't mean all 7447Bs are able to run at that clock frequency savely. And alway have in mind that overclocking provokes a shortened life span.