...but be prepared for a lot of controversy. A lot of Mac users steadfastly think that their Macs are invulnerable, armor plated computing monsters that nothing can hurt. They're wrong; the Mac platform and OS are both vulnerable, just not nearly as vulnerable as the PC platform and Windows OS.
And there are people who have had bad experiences with some AV software on the Mac platform, particularly Norton Antivirus for Mac. I don't know how they've had such bad experiences, since we've been running that software (always the current version) on my wife's G4 800MHz iBook since we got it and never had a bobble. (That's the kind of statement that starts arguments here, by the way-"I've always had it crash my Mac because..." or "it destroyed my Mac for no reason..." are the typical responses.)
But free is very good, so Clam, in concert with the OS X firewall, is a good choice.