Uh ok, fellow data security fiends. I agree with you guys. Really.
I would say it depends on what information resides in the hard drive though. If it's not anything important, I say a simple format will be fine. Or leave it alone.
If it's sensitive, or if you're security conscious, a minimum triple pass of random or zeros will do just fine. Unless you're a serial killer, Uncle Sam is not going to waste his time and money seizing your HD to be examined by specialized forensic scientists using an electron microscope.
Of course, you can also not sell it. Keep it, reformat it, and reuse it as an extra hard drive. This is usually my preferred method. Not that I have anything bad to hide, and yeah ok, I'm also paranoid by default, but I like recycling hard drives for extra storage anyway.
The caveman hammer method is really a last resort in my opinion. I've never smashed perfectly good hard drives before. I should also add that if you're the mob or harboring kiddie pr0n, Uncle Sam can still read HD fragments. Just FYI. There are more effective ways, but honestly, this route is just overkill.
Actually, if you have anything that sensitive, you won't want that kind of data to sit in the primary hard drive to begin with....