So I am Running PF sense on the following system's:
3x Celeron quadcore with passive cooling, 4gb Ram each, 120GB sata ssd.
Im running 3 of these with proxmox set up as failover and 4 gigabit nic's each.
1 for uplink1
2 for uplink 2
3 for Lan 1
4 for High Availability sync. (proxmox & PFsense)
You don't need anything fancy, but in my opinion don't Buy Cheap Nic's (if you are pushing a lot of traffic, these tend to burn out / overheat (especially if passively cooled.))
You can get a quad port Gigabit Nic from ebay for cheap if you are starting out.
Else I recommend getting a device from Netgate.
they are affordable and quite nice.