If you fancy nas, the terrastation has 1TB of disk space, this can be put in a raid 5 (giving 750mb usable).
http://www.buffalo-technology.com/products/product-detail.php?productid=97&categoryid=19 - infact it looks like they now have a 1.6TB version.
Disadvantage of NAS is it will only do file serving, if you want run stream serving software of anything like that it won't.
Best thing is to work of the rough cost of things you will need no matter which way you go (clients, switches, cabling), then compare the extra cost of a) a pre built nas (or several), b) a home built unit with enough performance for what you think you want to do now (celeron / semperon, 5hd, controller, gigabit, 512mb ram), and c) something with more power than you think you need (or the ability to expand to it, e.g. single xeon, ncch board, 1gb ram, 5hd, controller).
Either of the home built approches will give more power and flexability then the nas, the doing it as cheap as I can approach will save a little money (compared to the nas), the room to grow approach will cost about the same as a nas.
The client / infrastructure costs will be about the same no matter what you use, with regard to the two build it yourself approches, PCI-X support will add to the cost. But as so much of the server cost is disks and controllers it's liable to not cost that much extra.
Also, don't think because you have 5 rooms that means 5 devices. You may have a PC, a media center extender (or something similar), networked PVR, music player, laptops, pda's.
No matter which approch you take you will be spending alot of money, I would spend the extra to have more room the grow (and once you total up the cost of everything it may not be that much extra).
My point isn't that you need the power of the Xeon processors, but rather you wan't the bandwidth that at the moment is best obtained via PCI-X, and the NCCH is the cheapest board to do that, and it takes Xeons. Once an affordable PCI Express raid controller is available then you will have much more freedom for motherboards. Talking of bandwidth, no matter howmuch memory you use, you need it in 2 dimms to enable dual channel memory access (which doubles the memory bandwidth).