When I had set up my Watchguard Firebox x550e, I replaced the two 40mm fans with silent models. I also swapped the PSU with a 90W pico PSU to make a nearly silent system.
One of the replacement fans gave out and starting grinding a few weeks ago, so last week I replaced it with an HP T610 Plus coupled with an Intel i350 T2 dual-gigabit ethernet card.
Default Install pfSense 2.2 and no additional config flags. It runs at 17-19 Watts idle.
The T610 has 4GB RAM, 16GB MLC SSD, and an embedded 1.6 GHz AMD G-T65N dual-core processor. So it handles all my needs without breaking a sweat. 60 Mbit of VPN traffic (one way) barely even registers.
I haven’t done any iperf tests yet, but it should be equivalent to some of the dual-core Intel Atom boxes people use. So probably ~750 MBit and 150+ MBit over VPN.