I have been doing some research on which OS would be the best for a network device. According to Richard Bejtlich at http://www.taosecurity.com/, FreeBSD is a very good OS for this purpose, better than Windows or Linux. I'd like a device that will support running Snort constantly, with IPTables/Netfilter logging, and also support a webserver. I have tried running Snort on a P4, 2.66GHZ machine with 512 MB RAM with little success. The system was unable to process faster than 25 kbps. This speed is unacceptable with an average speed of 300 kbps on a cable modem. The OS that I used with this machine was Gentoo Linux, using the 2.6 kernel. The target machine for my FreeBSD testing is a custom job; Tyan Tiger MPX motherboard with dual Athlon MP 1200 processors, and 512 MB RAM. I am using 100BaseT Ethernet for all testing.
I will follow-up with the results of my testing, I am downloading the ISOs for FreeBSD right now.