And the 3rd-level-cache on the G4 is only needed because it doesn't have any other posibility to use modern DDR-RAM, which comes naturally for todays x86s.
You have it the opposite way round. x86 needs modern DDR-RAM for speed because of (bad!) legacy CPU design, while PPC is the opposite and is not sped up much by fast external RAM but is sped up greatly by extra cache. This is also why PPC is so good at cache-based calculation like RC5.
