https://www.pfsense.org/hardware/#requirements
Their recommendations:
CPU Selection
The numbers stated in the following sections can be increased slightly for quality NICs, and decreased (possibly substantially) with low quality NICs. All of the following numbers also assume no packages are installed.
10-20 Mbps We recommend a modern (less than 4 year old) Intel or AMD CPU clocked at at least 500MHz.
21-100 Mbps We recommend a modern 1.0 GHz Intel or AMD CPU.
101-500 Mbps No less than a modern Intel or AMD CPU clocked at 2.0 GHz. Server class hardware with PCI-e network adapters, or newer desktop hardware with PCI-e network adapters.
501+ Mbps Multiple cores at > 2.0GHz are required. Server class hardware with PCI-e network adapters.
The amount of traffic you're pushing is business class. You will need some moderately beefy hardware to manage it effectively.
Note: ClearOS is a pretty fair substitute if you prefer Linux to BSD.