I am pretty sure that Golem is correct. Everything I have read about the last PPC MacMini's has stated that the 1.5GHz PPC w/64mb of VRAM was a late revision to the 1.42GHz model. In fact, I have seen some sellers state that their 1.5GHz models came in boxes that had 1.42GHz stickers on them.

I don't know if that was the case for all of the 1.5GHz models, but it might have been true.