I second the one about the 500Mhz G3/400Mhz G4. The thing is, what do you compare when it comes to extra instruction sets? if it was doing media, i would say the G4. if it was just doing everyday stuff, i would say the G3. (G4 only trumps the G3 at it's speed when under OS X or doing media) Apple touted the G4 too much, but when the G4 came out, the G3 trumped it at slightly higher speeds in normal everyday use, because the G4's altivec didn't get used until OS X came out. Then OS X used Altivec for just about everything.
G3 was a nice CPU, but once OS X came into full swing, and media was just about everywhere, that's where the G4 showed it's true colors.
a 700Mhz P III can only trump a G3 if it's media related. Otherwise, what makes the difference is the Operating System. The P III has a better chance of trumping even a G4 if a system was compiled to take specific advantages of the CPU Features. Otherwise, as far as Windows? a low end G4 trumps a P III 1.2Ghz because of how slow windows crawls compared to OS X which hands off the window rendering to the graphics card. It doesn't bog down the CPU when you move the windows.
Vista is a bit better... I don't see streaking when moving windows around under vista, but it does on XP Pro. this is with the same video card (Radeon 9600 Pro 256MB AGP 8x) My B&W G4 400Mhz w/ 32MB PCI Radeon 7000 does a better job at rendering the windows than M$ Windows XP does.