If you want to erase everything so you sell it with a blank hard disk i recomend that you have a look at Boot and Nuke from http://dban.sourceforge.net
you create a boot disk and it securely erases everything on your hadr disk.
Fdisk will just erase the partiotion table.
formating will only erase the flie start information and re-write the TOC (Table of contents) the only way to truely erase a disk is to zero it, there are other programs available aside from the one I posted, if you do not zero a disk then data is recoverable using disk foresnsic techniques, you can actually use shopbought programs to recover information from drives that have been re-partitioned uing Fdisk, and recover information from drives that have een formatted.
The type of erasing you'll want to do may depend very much on the sensitivity of the data contained on the disk, for instance you may not want your work/buisiness information falling into the hands of whoever happsens to end up with your PC after you sell it.