Apple's builtin firewall is all you need. Go to System Preferences -> Sharing. If you want something a little more configurable, search for BrickHouse (I think that's the name). It's just an interface that allows you to configure more details of the builtin firewall.
As for anti-virus, I wouldn't bother. I have Virex as part of .Mac, but I never run it. There's just no need. I've read about a few proof of concept viruses/spyware, but that's about it. Nothing that's actually spreading in the wild. I'd hold off until news indicates that viruses are becoming a real problem on OS X. That may or may not ever happen.