If you are really REALLY worried, then clone your HDD to an external, boot from the external, secure erase the internal drive and reinstall your OS on the internal drive. Most people outside of MAYBE highly advanced data recovery specialists (and even then I think it is doubtful) will find any data on your drive.
Then just format and erase your external drive once you are sure you do not need anything from your old system any further.
If you have Lion or Mountain Lion, here is a very secure way to wipe your data.
Turn on Filevault encryption and wait for that to encrypt the entire drive. Now do a command-r boot to recovery. Then start Disk Utility and erase Macintosh HD, then reinstall the OS.
So you have erased a completely encrypted partition that nobody is going to be able to recover anything from.