1 or 2 passes is more than enough. Someone who buys a computer isn't the type that plans to hack old data from your period of ownership. Retrieving data from the former owner occurs most often with stolen computers.
Even if you did nothing and just re-installed OS X, it might won't matter because the new owner will probably create new files that overwrite everything you did anyway.
I myself have sold several Macs. Before shipping them to their new owners I only wiped the drive during the re-installation of OS X. I didn't bother with 1 or 2 passes to cleanse the drive, and I have never had any problems. Like I said, someone who buys your machine wants to use it, not hack your old files.