I have that NIC laying around, but after pricing a build up, I went with one of these ebay/amazon/alibaba fanless builds- quiet and no moving parts (using SSD), should be here in a month and will post up a review.
Made a thread asking on advice but got nothing- will have 4 gig RAM, spare 120gig SSD laying around, plan on running just a few things like Snort, OpenVPN to VPN into home network. Been googling if freebsd/pfsense figures out trimming on it's own or I have to mangle something together in the command line.
That build should be good enough for pfSense, as others have said just add network cards and ram and your set, I have mine running on a low power Athlon 5350 SOC and haven't had any issues with it at all.
FWIW - I didn't have a SSD when I built my pfsense box, instead I used an old 500gb HDD. Pretty much everything loads into ram; you can create ram disks for logs and var, so that even more stuff goes into ram. And then you can do automated cron dumps a few times a day, so write back to disk. Even though you have an ssd, I would look into loading more crap into ram. It seems to make things a bit snappier with my overpowered router.
I'd suggest to go with a CPU which consumes less power like the AM1 Athlon 5350 MichaelLindman recommended. I think you should look for used network cards from servers on eBay. I got a dual port Intel network card used in HP servers for 20€.