The safest bet is to scrub the drive before installing a fresh operating system. Hatta's method is good; it turns every bit on the drive into a zero.
Allegedly, someone who is really good at hard drive forensics cab recover data even from a drive that's been written over once. It's something to do with how the physical media that creates each bit is arranged after it has been written one way, then another; something of a "ghost image" of the previous data possibly can be reconstructed. Obviously somebody is going to have to really
want your old data for this to happen. If you really want to be that paranoid, the solution is to do multiple swaths of random data instead, using /dev/urandom and running the command multiple times.
As for a new OS install, depending on how you plan on selling the laptop, you may not have to worry about it. Techy types know how to install their preferred OS, so just advertising that an OS is not preinstalled can be good enough. If you do want an OS on the laptop, however, consider using one of the friendlier linux distributions, like Ubuntu. There are also *cough* ways of installing Windows that don't require the original system CDs. I haven't looked for Vista or 7, but for XP it's a snap. I wouldn't recommend this method if the next owner isn't going to know any better, but again, techy types won't care, or will know how to deal with it.
Edited by FujiSkunk, Fri Mar 9, 2012 3:44 PM.