You might want to consider this:
That model, depending on spec, is typically going to consume between 225 watts and 350 watts. If you are running the thing 24/7, at an electric cost of $0.13/kWh, that's around 2500kWh or $325 in electricity for the first year of operation. That assumes only 225 watts; the figure increases to a staggering $400/year if you're burning closer to 350 watts.
Buying a cheap old-tech crudbox for use as a server is always tempting, but may not actually work out to be a great idea when you look at the TCO economics. And you have to worry about stuff like whether the BIOS will freak at large drives, etc?
If you intend for this thing to be lit up for years, you might want to consider making a purpose-built rig. Even the modern Xeon systems idle at low power. We've been using E3-1230 based boxes here with carefully selected components, and the base system idles around 45 watts idle, maybe 90 watts running full tilt. Now if you're going to add drives to that, you do need to add maybe 10 watts per drive, so a fileserver might be eating 85 watts 24/7, but that's substantially less than 225-plus-40-for-drives. And a Xeon box is generally going to be overkill for FreeNAS. Lots of these guys around here are doing things like the E35M1 based platforms, or for a completely prebuilt box, HP's awesome little Proliant Microserver N40L.
p.s. You should be booting from flash, not from an OS drive. If you use an actual drive, you're using a valuable SATA slot and extra watts.
My suggestion? Spend a little extra now. The N40L is frequently on sale for less than $300, is fairly light on power consumption (at least compared to the beast you're looking at), is BRAND SPANKING NEW, and in six months when you've recouped your additional cash outlay, you can start buying yourself a case of beer every month with what you're saving in electricity.