Everything runs better on a G4, my friend. Well, unless the G3 is significantly faster than the G4, like a G3 @ 900MHz vs. a G4 @ 400MHz -- obviously, the G3 will outperform the G4.
I think VirtualPC has some AltiVec support in it, so it'll run a bit better on a G4, but even if it doesn't, you're going to see better performance out of the faster processor, whether it be a G3 or G4.