My suggestion, buy the new hard drive install it as slave, keeping the old in the computer as master, then clone the old disk over to the new. Using something like Acronis true image. Then take out the old and set the new one as master. That way they won't need to reinstall and lose all their software, file's and what not.
You can do it in the trial version of Acronis so you don't need to buy the software. I did this very same thing on an e-machines (5+ years old) running ME.
Don't wait for the hard disk to fail, do it as soon as possible.