Erasing a computer before selling it

I will soon be selling my old iMac (first-gen Intel).

I booted it from the Snow Leopard drive, went to Disk Utility, and then did a 7-pass secure erase of the internal hard drive. It took many hours.

This should be safe, right? How can I be sure? Can Disk Utility really be trusted? Has someone actually done some research into the security of this?

Also, I have reset my firmware password to none. Are there any other things I should consider before selling the computer?