Unless you factor in the Altivec unit, which of course is a factor for some number of applications, the 750FX and 750GX are substantially faster than the G4.
If the code is special in that its perfect for a G3 and bad for a G4, and if the moonphase is at just the right level...
For the record, I love the G3. It's fast, and it runs cool, and I'm in no hurry to upgrade. But in no way would I consider it ever to reach the performance of a G4.