There are any number of vendors who will sell you a generic NAS platform, ranging from IBM/HP/Dell on the "big iron" side of things, to iXsystems, who will sell you their own variation of FreeNAS on what looks to be a Supermicro platform.
In general, NAS vendors build tightly-coupled offerings. On the low end, you have stuff like the iomega ix2 (
http://techcrunch.com/2012/06/11/up-close-with-the-updated-iomega-ix2-nas-drive/ ) which sports a (haha) massive 256MB RAM and a Marvell 1.6GHz SoC, and being an ARM platform, that makes it incompatible with FreeNAS. You won't be building a FreeNAS system that can touch its small footprint and tiny power consumption, but almost any FreeNAS will outperform it. There are many Atom based NAS boxes, such as the Sans Digital AccuNAS AN4L+ (~$400) but you can make a mini-ITX box yourself for less, or better yet, run out and get an HP Microserver N40L ($350 from HP Business) which lets you use more RAM *and* has expansion options. And if you want big iron, well, the NAS-vendor-supplied platforms are going to be pricey. Better to go buy a Supermicro SuperServer and build your own.