Destroying your files is a single step, well-implemented in Mac OS, and you don't even have to do this if you simply install a fresh OS and use the right settings
I don't think installing a fresh OS will remove personal data sufficiently. But maybe there is a setting to do a secure clean?
There's a secure wipe option. You can crank it up to some absurdly paranoid set of passes, although it takes a day or two to complete at that level given a reasonably large drive.
A day or two is fine. I had a friend who would go through old donated computers to get data off of them. I think the odds are pretty high that people will try on a given machine you give away.
