I remember looking at that exact thing, I think. The price is around $300 though, maybe $270. But definitely no lower than $270.
I ended up building my own system using an Intel Atom board/processor running Linux (Ubuntu) and a SATA PCI card that can connect four drives. Right now I have four 1.5TB drives connected to that card and a single 500GB drive for booting, since you can't boot off the PCI card, and definitely not off a RAID array you build. A friend helped me with all this; he kinda talked me into it too, and I don't regret my decision. I think of it as my big jump into Linux.
The board and case (including power supply) cost me about $150 and the drive about $60, though nowadays you can buy drives cheaper than when I built mine about eight months ago. The card cost like $30. RAM (1GB) cost $50-ish, I think. Going by memory here.
So if you know a place that prices their computer stuff fairly, and kinda know what you're doing, I'd point you in that direction instead. Not to mention the Intel NAS unit may be just a NAS with nothing else, so you might have trouble running anything on it. The Atom is also cheaper and runs on lower power. I even dare say it's got more processing juice than the Celeron. And you can install whatever on it - the Atom board should have one DDR2 slot so you can install up to 2GB of memory (1GB is plenty enough for Linux). There's nothing on the item description of the Intel NAS unit that says anything about being able to run third-party software on it.
So, if you're just looking to expand your storage space without wanting to seed off the NAS, you'd be fine with just buying that. It is a tad more expensive, but the only work you really have to do is open it up and install your four hard drives inside. Of course, that doesn't seem to be what you're looking for (you said you wanted to seed off of it).
If you're looking for more functionality, you're probably better off building your own.
BTW, if you're going to install four drives, consider creating a RAID 5 array. If you're going down the Linux route, let me know. I can help you a bit with that.