I don't think all that much, because even if the G3 had 1 GHz (which they say is going to happen), it wouldn't be as fast overall as the G4 at the same clock speed (altivec). By making faster G3s, it will give Apple greater longetivity to the iBook, allow for a cheaper consumer level laptops, and keep IBM supplying Apple with processors well into 2003.
Once the G5 comes along, things might get tricky. And I wouldn't be surprised if this hasn't been something of a factor in why we haven't seen it sooner. For Apple to have 3 different processors would be inefectual, they'd end up with the same problem they had with the cube (maybe).