I typically remove the hard drive. Alternately, I have used
dd to copy
/dev/null over the drive. If I had known about
shred then I would instead have done
from a rescue disk, but I only learned about that command in someone else's answer above. Hard drives are cheap enough, however, that I have typically just removed and disassembled the hard drive. (I like strong magnets, grin.) Since I'm not worried about the NSA trying to recover data from my drive, just wiping with
dd if=/dev/null of=/dev/sda
dd if=/dev/null of=/dev/sdb
from a rescue disk is sufficient for my worries.
There is no reason I can think of to cleanse anything other than the hard drive. However, I will sometimes reset the BIOS back to defaults just to put it in a known state for the next person. In case, for example, they want to change the RAM and my overclocked RAM settings don't work for their new RAM.