No need for an SSD for FreeNAS: you can install the OS on a 2gb compact flash card and use a compact flash to sata adapter to hook it up to your motherboard. I use
this one, it works great!
Also, have you considered using an Atom motherboard for your NAS?
Just so you get a good idea about performance, I run FreeNAS off an Atom330 mobo/cpu with 2gb of DDR2 ram. This little combo is powerful enough to handle 4 drives in Raid-Z with power to spare!
The only downside to mini-itx boards are the number of sata ports, but with the PCI-E 1x slot, I recommend using a 2-4 port SATA controller.
Don't go overboard with the hardware, FreeNAS requires little power. Save some of that money to invest in drives!
My .02!