Yes, I think the lower the computer perfomance per core, the higher the boost going from XP to Vista/7.
If a crap computer goes from 20 to 30 it means 50% boost
If a nice computer goes from 40 to 50 it means 25% boost
And so on.
But anyway, I suppose there are many other variables that can affect that. In anycase, that upgrade is like putting an new cpu in your computer.