You want to not only erase your data or format the disk but make sure that the entire contents of the disk are overwritten with new information. Apparently you can do this by booting off of your original Mac OS X CD/DVD. Have a look at this page:
Go to the "Steps for zeroing data" section under "Cannot zero the startup disk".
When done reinstall Mac OS X again and it should not only be a clean computer but one with no data left on it.
I know people who would repeat that process several times...
I also know people who swear that the only totally safe way is to smash the hard disk open and take a belt sander to the actual disk...
It's true that the only totally safe way to erase all information is something like sanding the actual disk, but that's an excessive standard of safety. A sensible standard of safety is making recovery of the data too expensive to be worthwhile for a potential attacker.
Zeroing the data is entirely sufficient for normal purposes. It may be possible for a suitably equipped lab to recover the data after the disk has been zeroed, but that's too expensive to for anyone to do unless they know that the data on the disk is worth the cost.