In terms of (yes just adding to my post) - the processors from Intel now in Intel macs are the same as you will find in PCs, no Apple optimisations.
The big difference between PPC and Intel processors used to the pipeline stage; with the Pentium 4 -- Intel created a processor that could have its speed ramped up by great levels to start with (as they hit the buffers with the Pentium III Tualatin core...), but it had a long pipeline.
The G4 (which is composed of many different types of PPC chip) had a shorter pipeline initially; meaning instructions got to it quicker, and thus despite the MHz rating being slower, it made up for it with its short pipeline.
The Pentium M (Banias, Dothan, Sonoma and now Yonah/Intel Core) design is an extension of the Pentium III design which was a faster chip per clock cycle than the Pentium 4. A PIII-Tualatin at 1.2GHz would match a 1.8GHz Pentium 4. Essentially the technology evolved enough to really take the PIII design beyond the roadblock.
As noted though, the G4 processor vary quite a bit, within the 'G4' family are the PPC 7400, 7410, 7450, 7445, 7550, 7447, 7448 (and probably others I forget) processors that whilst more or less the same, some have quite substantial differences, such as between the 7400 and 7410. Wikipedia explains this, and you will find that to ramp the G4s speed up, Motorola (Freescale) made the pipeline stage longer amongst other sacrifices...
Vicky