try ClamXav click here . this is a port of clamav for linux and should do just as good a job as the linux one. i use the linux and windows port of it and highly recomend it as it is opensource and has a exelent av definition base updated several times a day on ocasions but usualy only once.
you should defonatly run a firewall av is not so nesisary as there are so few viri for the mac as with linux but hackers are still a big thret on linux and mac to. does mac use a root password and low level users like linux out of interest?