From what I can tell, and there is no real testing or scientfic analysis to support this....only my experience and opinion...if you take the Mhz number from a mac Gx chip and double it, you should come close to what the
Intel equivalent would be.

for example:
a G3 400 is just about the same as a 700 to 800 Mhz Intel
a G4 500 would be about the same as a 1Ghz Intel
a G5 1.25 Ghz would be around a 2.5 Ghz Intel

you get the idea

This wil also correspond with the release dates of each processor. G3 400 was current around the same time
that Pentiums were just shy of reaching the 1 Ghz goal...around 1999 to early 2000.