lol. its true though. you could sell it, but (if youre paranoid) there are simple utilities that can allow the buyer to see what youve deleted, since deleting doesnt really delete your old files, it just no longer allows access to them, getting rid of individual file or folder icons on screen.
The best thing to do (if its not too old or too small in memory) is probably to install your old hard drive on your new computer as a second hard drive, eg where to install non important music files, or where to keep secondary backups.. if you dont want to do this, then go with your first reply instead! ;)
I would keep the hard drive in house, you never know what unconciously you may have put on it years ago, photos of your children etc. I just bought a box from maplin to put a drive in converting it to an external hard disc, about £20 including all the leads & connectors.