Hardware Requirements

At the minimum, your host machine for OPNsense should have the following:
1GHz Dual Core CPU
2 GB RAM
The recommended hardware, per OPNsense official documentation is as follows:
1.5 GHz CPU
8 GB RAM
120 GB SSD
This recommended set up will allow you to utilize the full functionality of OPNsense, which includes IDS and IPS software if desired. My host machine, a Dell OptiPlex 7050 SFF has the following specs:
Intel i5-7600 @3.50 GHz 
8 GB DDR4 RAM
512 GB SSD 
Additionally, your host machine needs at least 2 RJ45 ports, 1 for your WAN connection and 1 for your LAN connection. My OptiPlex only had 1 RJ45 port, so I purchased an Intel I350-T4 NIC and installed it onto the motherboard, which gave me 4 RJ45 gigabit ports. More information about that can be found here.