I struggled with this for months. Here's what I did and why:
I already had an old northwood 3.0 and raid capable motherboard for which I found a new bios for that supported raid 5. The board has a GB ethernet NIC integral to it. I started out buying a case ($59, 7 x 3 1/2" internal, 5 x 5 1/4" external) and a tagan 700W single 12v rail PS ($80). I bought 3 x samsung spinpoint F1 750 GB sata II ($99 ea). Put xp on it, built the array with a stripe size of 32k. 15 MB/s write, 50 MB/s read. Pathetic!! Pulled out a drive, formatted it on another PC, and stuck it back in for a rebuild of the ~1.4 TB array. After running for 4 days and not completing, I pulled the plug.
Phase 2 additions:
Intel DG33TL motherboard ($99). Integrated video, HD audio (digital in, digital out), Intel GBe, ICH9R onboard. Intel e7200 CPU ($99) - 65W TDP, 4 x corsair XMS2 2 GB PC6400 DDR2 ($55 per pair - total $110).
Bought a ocpy of vista64 to address the 8GB ram AND create GPT > 2TB. Bought another samsung spnpoint F1 ( so now 4 x 750 in raid 5) in 1 ~2.1TB volume.
I used passmark to benchmark seq read, seq write, and random access locally. Over 200 MB/s read and write locally. Random access ~10MB/s. I used the same Intel NAS benchmark suite and got over double performance in every single benchmark over the network that the reviewed NAS got. Rebuild of the array takes ~4 hours.
Approximately $850 + OS. Found a hack to put the terminal services DDL from server 2008 on vista64 sp1 so I can have multiple simultaneous users remoted-in. I have MySQL, SQL Server 2005, and SQL Server 2008 installed, antivirus, centralized backup of all PC's on the network to one location.
In my opinion, unless it is REALLY CHEAP, NAS is a waste of money. If you can administer this NAS (raid level, permissions), then you can administer the "server" I describe with the added bonus of being able to use a windows interface.