I don't think trinux would be the best micro distro for routing, its really a security toolkit - here's what they say on the trinux site (which is now http://trinux.sourceforge.net/ )
Trinux contains the latest versions of popular Open Source network security tools for port scanning, packet sniffing, vulnerability scanning, sniffer detection, packet construction, active/passive OS fingerprinting, network monitoring, session-hijacking, backup/recovery, computer forensics, intrusion detection, and more.
I'm confused how a 486 with 64meg of ram caused your connection to slow down hantiz, for plain vanilla routing i've used a 25mhz 386 in the past without any sort of latency - although in answer to your question i would recommend a 486 with 32meg ram to do any decent firewalling. I use debian because thats what i like, but its much of a muchness what distro you use as long as you keep it up to date and locked down.
Can't answer your freebsd question tho - don't use bsd's.