If you have a Linux liveCD, you can generally do a Good Enough(tm) job of wiping the data via:
dd if=/dev/urandom of=/dev/sda
which covers the entire drive (including partition table) with random junk. It's theoretically possible for a hardcore data-recovery specialist to get your data back after that, but if you're trafficking in data that's sensitive/important enough to warrant it, chances are you won't be allowing the disks outside of your extremely-secure armed-guarded facility to begin with, at least not intact.
However, the safest way to ensure that a hard drive won't have any information that gets into someone else's hands is to simply not provide the computer with the hard drive. Pull the drives and donate everything else; hard drives are cheap and unreliable (i.e. failure-prone) enough that anyone who wants to use the computer will probably be better off buying a new drive anyway.
This way you'll also still have the drives in your possession in case you discover that some of the data on it actually was quite important. (I highly recommend labelling the disks after you pull them so you can keep them straight for this reason. Obviously you do NOT want to wipe the drive first if you're going to do this...)