Several more options, in case you have not already selected one of the others.
1) As mentioned, you could connect your 'target/destination" Mac via Firewire cable to another Mac running OS X, and use the second Mac DiskUtility to Format with Write Zeros (as many times as believed desirable) onto the target Mac HD. This of course destroys the OS (Operating System) so the machine will not boot again until someone installs OS X again.
A PC will not recognize a Mac in this manner.
If you want to keep the existing Mac OS X, download one or several of the secure erase programs. As you probably already know, merely deleting a file by throwing them in the trash does not delete the data, it just removes the directory name and data pointers.
2) Erase file by secure delete after dragging them to the trash.
If you want to keep the existing Mac OS X, download the free Permanent Eraser from Edenwaith. http://www.edenwaith.com/products/permanent%20eraser/
It runs a very secure -and very long- delete process.
3) If you have already deleted some or most files by Trashing, use one of the other tools to also erase/overwrite free space. (free space of course includes anything that you ever had on the drive but then trashed.)
- Trash X from Norther Softworks. $9 or so, but has 30-day tryout.
- ShredIt X from Mireth. $25.
and some others.
Note that if you use a non-Apple method to totally erase the HD down to low level, it may also remove the protected Apple machine serial number... if that is important to you.