I am selling myiMACG5 and want to erase/delete all data on the hard drive.
Boot from the installer disc. Erase/format the drive. Select write zeros once in security options to make sure some cannot easily recover your files (unless you really don't care). Writing once is usually sufficient and will still take a long time (hours if you have a very large drive in there; days if you selected the top level option).
Re-install the operating system after wiping. Make sure you give the buyer the original discs that came with the computer (they do you no good and you can't sell them) and any installer discs for any upgraded operating system and software you leave on the computer (required by license).
Kappy's extensive hard drive formatting post - http://discussions.apple.com/thread.jspa?messageID=8340551 - note that instructions at end are for zeroing data.
Pondini: Formatting, Partitioning, Verifying, and Repairing Disks... - http://web.me.com/pondini/AppleTips/DU.html