In my opinion, any core 2 is superior to a P4. As soon as you try to do two things at once you'll notice the difference in the core 2 processors.
If it's going to be a significant difference depends all on how the user's using the computer. If they just poke around on IE and look at facebook, there won't be much difference. If the do real work on the computer I think they'll be thankful for it.
But it's a different hardware architecture. You will have to reinstall windows and your programs.