I had this debate with meh IT consultant recently (Thanks DD, btw). I store fairly sensitive personal data belonging to thousands of clients on multiple PCs. The consensus was that there is NO secure erasing technique - it just depends how badly someone wants your data.
The odds are it's not too important and no-one wants it. So the kid next door can't get it, and the NSA/GCHQ don't want it. However, I have a duty of care to make sure no-one gets it, so out came the pick and 10lb sledge and 10 HDD's went "blam".
Wear eye protection