Hi!
If I recall right HP sell a firewall server for Microsoft ISA, the hardware is a HP DL320 and the CPU are Celeron, that why I think that for pfSense I am sure that the Celeron must be more than enough.
And for the NIC the recommendations is a good quality net cards (right now Intel or Broadcomm chipset, you get probably VLAN support, QoS tag, TCP Offload, etc.) and if possible all the cards must be the same model/brand.
Greetings…