Boot from the original install DVD and launch Disk Utility from the Utility menu when the menu bar appears. Then select the hard drive and under options choose the seven times erase — this will take a good while to complete but short of a top notch computer forensic lab nothing on the drive will be recoverable. Then go ahead and install the operating system.
At this point your data is secure and the computers are usable by someone else. As you say they can no longer be upgraded, but there are likely agencies that would be grateful to have them. In our city the Salvation Army will take and resell them to people who cannot afford anything else.
