Lowest power usage build? (Smoothwall box)

I really want to put together a low powered Smoothwall box and get away from the P4 system I currently have my SW3 running on. Currently, I'm looking at this board here:

Intel BOXD945GCLF Intel Atom processor 230 Intel 945GC Mini ITX Motherboard/CPU Combo - Retail

It has a built in 10/100 NIC which I can use for the WAN, and it has 1 PCI slot, which I can use for a 10/100 NIC for the LAN side. It's also a Mini ITX board which would allow me to make a very small custom case for it and not have to use a regular full size case as I currently have to for the boards I do have on hand. But is this really my best option for a low power consumption PC, or is there better? Keep in mind, this will be strictly used to run Smoothwall, and I must have at least 2 NICs available (either built in or through a expansion card [no dual port NIC cards!]).

It is probably you best option. Next in line would be a 35W Celeron-L. It would be better if Intel would use more modern chipsets instead of the old 90nm heap in it now.

i have one of these:
http://www.via.com.tw/en/products/mainboards/motherboards.jsp?motherboard_id=81
coupled with a 4gb cf card and a dual port pci nic, it consumes all of about 20 watts

"Our test motherboard consumes 59 W in standby, and we reached 62 W under maximum load (with a 3.5" hard disk and a 1 GB DDR2 DIMM). Obviously, these values are what we measured for the complete platform, not only the motherboard, and they don’t take power-supply losses into account (our test model has a yield of approximately 80%). That’s both a little and a lot – it’s not much for a desktop computer, of course, but it’s a lot in absolute terms. We should add that we recently tested a motherboard based on a 1.5 GHz Via C7, and the configuration drew less power with the same components: 49 W at idle and 59 W under load (always measured at the AC outlet)."

http://www.tomshardware.com/reviews/intel-atom-cpu,1947-7.html