antivirus: my company uses both clamav and bitdefender. This month so far clamav has caught 603 viruses and let 10 pass
which were caught by bitdefender. (I just hope that 613 were all there were...)

firewall: go to www.netfilter.org and get (at least) "Packet Filtering HOWTO". Then build your own firewall. There are
prepacked solutions out there, but I feel that there is no substitue for really digging in and understanding iptables/netfilter.
YMMV, of course.