Just use the built in firewall and switch off any services you don't need (like Personal File sharing, FTP, WWW), then just not bother with Anti-Virus.
For the moment there are naff all virii about on the OSX platform to bother you (they do exist but only as proof of concept stuff I believe), the only reason you might want it is for filtering out Windows virii when you get them via email or in macros form with Word/Excel documents .. basically if you buy Norton Anti Virus for OSX it is to clean up Windows virii ... who cares! :-D
