I like Duckie's entry. The average person looks at the Ghz and how BIG the HDD is and that is about it.
Case in point. I was trying to get my uncle to buy an SSD over simply getting a new HDD for his new build. He doesn't use hardly any storage so size means nothing in his case. His new woman starting asking me questions, and the first thing was how many "Gigs" is is. Just a general "Gigs", not memory or HDD space or whatever, just out of nowhere, "how many gig's is it though?". I asked how many gigs of what, and her response was "you know, the size of the tower's GB's". I knew she didn't know what in the world she was even talking about at that point so I went into some teaching for them both. About half an hour later, I thought I had did a pretty good job of explaining why this was a better choice. I thought wrong. After ordering everything, it seems he had canceled the SSD and put an order in for an old 500GB Green drive, 5400 RPM even. It seems after I had left, his woman had said she knew what she was talking about and that the 500GB was way better than a much smaller 128GB one and actually convinced him to change it.(Keep in mind he has an older 7200 RPM/320GB drive for storage already and is probably faster than the 5400 one, even though it was older) Apparently she has a son that "knows about computers" and he said go for the HDD so he listened. The sad part is, he knows that I know what I am doing and usually follows my advice. I'm not sure if it was him not trusting me anymore or more just trying to please the woman, though I am leaning toward the latter in this case.
Long story short, Ducky is spot on. They don't need some long winded analogy, just tell them it has more Gigs or GB's and they will buy it