Wwiping the ibook G4 clean for sale

If you are worried just buy a new/used hard drive for it and then install a fresh OS on that and keep your current drive, that being said if you don't want to buy a new drive you could always zero out the current one and install fresh on that. In my opinion the peace of mind that makes the 2% chance that someone could find a way to get your information off an old drive as opposed to a 0% chance with a new hdd is worth the price of the hdd.

Formatting the hard drive with the "zero drive" option, which basically does exactly that -- writes zeros to the entire drive surface -- is sufficient to protect from any sort of data theft. You can even have Disk Utility do a 35-pass format if you have CIA secrets on your computer and are extra paranoid! No need to purchase a new hard drive, and the probablilities given by cyclingplatypus are just silly, made-up numbers.

You will need an OS X installation disk however. While booted from an installation disk, select disk utility from the menu (which depends on which version of OS X you're installing, but it's very easy to find nonetheless), select your hard drive on the left in the Disk Utility window, click on the "erase" tab, click the "security options" button and choose to zero the disk. Click "ok' and then click the final "erase" button to do the job. After that, continue with the installation of OS X.

Wiping the drive currently in the computer also means that you don't have to do major surgery to get it out.

Zeroing will do the trick, but as aqua pointed out, if you're feeling paranoid, do the 35-pass.