Hard drive overwrite

I'm selling my iMac G5 (isight), it having developed VRAM problems. To remove data, I understand that overwriting prevents data recovery and would be safest. Do I do this by starting it up in target mode and overwriting from another machine, then start up the iMac from the instal disk? As it's a bit final, I thought I better check? The G5 is still working but has the dreaded green wavy lines. Many thanks.