Saying that a G3/500 would be faster than a G4/400 in non-Altivec apps isn't as straightforward a statement as it seems. The G4 introuduced some new efficiencies to the core that make it slightly faster than a G3 at the same speed even without Altivec. A G4/400 won't run even with a G3/500 but it won't be as slow as you would think going purely by the rated speeds. I still think the G4/400 is a better upgrade option.