I have a MacBook running Mac OS X Leopard, version 10.5.1.
I don't run any 3rd party security software, but I keep an eye on process running. AFAIK, the current number of viruses in the wild capable of infecting Mac OS X is a big fat ZERO, and the rest of spyware and malware is easy to avoid.
For security, make two accounts. An admin, and a normal one for everyday work. You don't really have to log into the admin account very often... Mac OS X asks for the admin account username and password as it needs it. Just be smart of when you actually type in this password, such as when you click a link that you don't expect to be a download of an application turns out to be one and wants your admin account...
And only download programs from trusted sites.
Also, I use password savings sparingly, and turned off the "auto fill online forms" tool in browsers, to avoid personal data (credit card # and such) being saved.