NAS vs PC, why

sorry for the spelling, i'm dyslectic.  

Why should you buy one of the pre-built devices instead of building your own? Some would give arguments based on size or the supposed criticality of saving every watt and never turning off the device, and of course the convenience of technical support and having the device pre-designed, built, and largely pre-configured for you. I'm not in that camp, and tend to encourage enthusiasts to build their own machine for better value and performance. Of course the reverse point of view applies to many others -- those who don't have the knowledge or time or inclination.

I wouldn't make a couple of design choices that you've made -- (1) PCI storage controller and PCI gigabit (in both cases, even the on-board one here goes through PCI) (2) Sticking with old DDR (3) Using a very old CPU socket (the AMD).

I have nothing really against DDR and old CPUs in this application; it's just that you get many more options and often better performance and support if you don't pursue obsolete technology, and it's much easier to solve (1).

Native on-board gigabit and SATA (at least 4) should be fine in this application. Not having to buy an add-on storage controller or gigabit NIC also gives you a bit more money to compensate for the old RAM. Using an old/junk PCI video card could give you some more options in motherboards (esp. for more on-board SATA). I bought 10 on eBay for $5 (plus reasonable shipping).

Note also that drive prices have been coming down fairly significantly, and it's possible to do more with fewer drives than before.

the NAS side of thing has sprung up recently, if you have that many drives and they are SATA i would recomend using RAID it will take a bit longer at first but trust me it worth only having one big drive then lot of littles drive.

also look into Lin NAS and the BSD free NAS