This is the motherboard I am using, but I have 32GB of RAM installed.
It's an excellent unit, works with FreeNAS out of the box, has a USB header directly on the mother so you can slot a thumbdrive directly in for your FreeNAS install, and the IPMI feature is sooo nice. In total, motherboard, plus CPU plus 32GB of RAM, I probably paid 350.00, but in UK Sterling. I know I have four times the amount of RAM, however, depending on what exchange rate your looking using $400 seems a pretty good deal.