you might wanna check out some tools for your setup:

1. tripwire for your firewall
2. inflex for sendmail ( a mail-relay is important)
3. jdk if you've got an extra old 486 or somthing ( a little IDS)
4. swatch (log file watcher)
5. a good antivirus ( I prefer norton myself, but any well known will do)
6. MRTG (a great (the best !) tool for monitoring router usage and any snmp oid for that matter).

I'll write some more once I remember them