Consider also the technique used in the Norton Utilities program "Diskwipe" which takes a /g switch which "Follows certain government rules for wiping". I can't find the manual but I think it specifies how many repetitions are used and different values to write in each.

The Norton Utilities Wipedisk /g does three passes writing 1's and then 0's, it follows this with the hex 'FF'. This shoud pretty much remove any residual traces of the data previously held on the disk.