I agree with the others who like the Intel Atom C2758 (Rangeley) or similar.
I'm also going to build a dedicated firewall and will be making use of the new Atom.
It's one of their "7-Year product life", so I know their support will be better and ease of getting parts later down the line than most.
I'm just waiting for the next major upgrade of pfsense.
I generally avoid Realtek NICs, support has always been abit crap at best and performance about the same, but maybe that will be different with the next upgrade.
I did considered the older Atoms, but I realised they were just powerful enough for my means, but were anywhere from 2-4 years old now and still costing me a similar amount as the new Intel Atom (I live in England,UK). Why spend money on a dedicated hardware for a firewall, if it's not overkill.
You need and want it to last, you need to be able to get replacement parts and the cost is not that different, also with all that more power any packages you want to use, you have the freedom to use. You never know what the future might require these firewalls to do.