ped said:
There is a difference - Ghz is not necessarily the same between the two. The difference is in the architecture of the CPU and with how much each can complete in a clock cycle. Apple's CPUs are RISC-ish processors and generally get a bit more work done per clock cycle than a CISC chip like a P4 does (although thr P4 has some semblances of RISC in it too - the definitions have become blurry over the decades as each have adopted techniques of the other).
Actually they both have a RISC core the difference is that the Pentium runs a sort of x86 emulator (in hardware of course) and has a deep narrow pipeline. The PPC is a shallow and wide pipeline (except the G5 which is deep and super wide).
The Pentium M which is probably the nearest Intel chip (clock for clock) to a G4 is narrow and wide as well but has a high density Pentium 3 core (remember the G4 was the original P3 beater when it was clock for clock).
What this means is that a Pentium M at 1.6 Ghz is probably around the same performance as a G4 at 1.5 Ghz but has a faster FSB.
The only way of doing a proper comparison is with code optimised for each CPU which does exactly the same tasks, not easy and IMHO something that has never been done.
The G5 and Pentium 4 extreme just cannot be compared in any meaningful way and currently there are no 64 bit optimised applications for the Athlon 64 and very few for the G5 so that is in a similar no-compare situation.
Custom PC magazine this month did a massive comparison across all current PC CPUs and they are all much of a muchness. i.e just to pick one category at the bottom of the results came the Celeron 2.66Ghz and the 1.6Ghz Pentium M with a Doom 3 frame rate of 48 ish and top of the pile was the Athlon 64 FX-55 with 80 ish (all tests ran with the same hardware on the PCs with the same game settings). Other test categories like video compression had results that were farther apart with the Athlon being about 3x the M and 2.5 times the Celeron, it all really depends on what you are using the CPU for.
Personally I find my twin G5 to be blindingly fast at everything I throw at it and although I don't play Doom 3 the games I do have for it will easily top 100 FPS with the options at maximum (Medal of Honour for example).
Amen-Moses