You boot from a USB drive. I think you only need 4GB. I went with overkill and got this:

http://www.amazon.com/gp/product/B00LLEN5FQ?psc=1&redirect=true&ref_=oh_aui_detailpage_o03_s03

If you want to use ZIL or L2ARC you'd need an SSD and I'd suggest reading a lot: https://forums.freenas.org/index.php?threads/slideshow-explaining-vdev-zpool-zil-and-l2arc-for-noobs.7775/