My wife's in the process of revising a couple of her music theory workbooks at the moment. Her workhorse machine isn't really up to the task of running InDesign, Sibelius, Acrobat, Outlook and Word with its half GB of memory. Her system is paging a bunch of things on and off the HDD. I explained to her that even though she has 50+ GB free on her hard drive, she's run out of RAM. Of course, a glazed look greeted this mumbo-jumbo and I was forced to reassess my approach.
I told her that the RAM was like her brain, she could only remember so much at any one time. If she wanted to remember other things, she could write them down in a book and put that book on a shelf. In this case, the shelf is the hard drive. To remember that thing, she'd need to go to the shelf and read the book, which takes much longer than just recalling it.
She was pretty chuffed when I told her that by adding more RAM, we're going to increase the number of things she can remember without having to write them down.