Wiping existing data with null data ("zeroing out," or sanitization) is sufficient for nearly all businesses and people. A 3-pass erase meets DOD/NISP 5220.22M specifications for low level classified material (i.e., not classified secret or top secret).
It takes a lot of time and money to extract data after a 3-pass erase, and I doubt you have data that is that sensitive. If it's that sensitive, then you're best to 35-pass erase and destroy the hard drive. S and TS material requires not only a 35-pass zero erase, but also physical destruction of the hard by shredding and then melting the drive platters. People can still read some intertrack data from a sanitized disk, and sanitization doesn't erase residual data that resides in bad tracks/segments.
Keep in mind that a 7-pass erase will take about 12 hours on a 250 GB hard drive. A 35-pass erase takes days.