In the vast majority of tasks altivec is not going to make up for 500mhz. Though, make note of the fact that the G3 had a very week FPU, and at 1ghz would properly perform like a 500mhz G4 for media tasks. But I doubt much slower. So the G3 is really a better deal, in some things it will be sigificantly faster because the G3 did have a strong ALU.