My pfSense router is built out of an old HP 6000 Pro machine, it is configured as follows:

• Pentium E6300 2.8Ghz (dual core)
• 2GB RAM
• 160GB HD
• Onboard Intel Gigabit and 4 port Intel PCIe Gigabit NICs
  1. WAN Interface – CenturyLink 1gbps FTTH
  2. LAN Interface – 192.168.1.0/24
  3. IoT Interface – 172.16.0.0/16 (internet only)
  4. CCTV Interface – 10.0.0.0/16 (only access to NVR)
  5. Webserv Interface – 10.100.0.0/16 (web facing servers)
• QOS priority on VoIP

I was worried this old machine would be a bottleneck when we upgraded to gigabit service, but this is still performing quite well even at gigabit service speeds.