The extent you go to depends on how much you really need to ensure the content of the drive is irretrievable. If you just want to do the minimum, then reformatting and/or using disk eraser tools do the basics. If you're serious, then removing and physically destroying the hard disk (hammer or drill is a simple but effective way) is the only way to ensure its contents are gone. Hard disks are not expensive, so I wouldn't trade my privacy over worrying about leaving a usable disk, if I were you.