Overwriting all files would be the best bet, but it too has its flaws, if you read the description in the bottom paragraph of that chaos shredder's site it says "CHAOS Shredder will completely erase files from your disk, without the possibility to recover it by any practical software or hardware methods." Recovering these files is still possible, i have no clue how, but i know it can be done. The only way to permanently get rid of all your files would be to have the person you are giving the computer to buy a new hard drive, then remove yours, and take the top cover off and take a torch to the plates.