Hey there!
Here's a NAS that I built last month, out of a computer that I was given, I had to do some modifications to it (replacing the Thermal paste on the CPU heatsink, replacing the RAM)
The following picture is old as it was taken before I added the other HDDs and the Graphics Card
Edit
However, here's the specs of the NAS
Motherboard: ECS GeForce 7050-M (Socket A 462)
RAM: 1GB DDR2 (the 2gb stick died) 3GB DDR2 (soon upgrading to 2x4GB)
HDD 1: Western Digital Caviar 120GB PATA (System Drive)
HDD 2&3 : Hitachi Deskstar P7K500 500GB SATA (data storage)
CPU: AMD Athlon 64 X2 (Brisbane core, 5000+)
Graphics: Built-in Motherboard chipset (formerly Radeon HD5450) Generic Graphics PCIe card
PSU: Allied 400W PSU (I am currently saving up to replace it)
Operating system: OpenMediaVault 4.x Arakis 3.0.83 Erasmus
Miscellaneous : Teac 3"½ Floppy drive (experimenting with boot floppies, such as KolibriOS)
Currently I use this NAS to store my backups (the two storage drives are mapped via SAMBA) I'm not really familiar with UNIX systems so I had a little trouble configuring it (installing OMV-Extras from APT-GET). I had to reinstall it 4 times (because it would sometimes get stuck at "Raise network interfaces", power-cycling it would sometimes fix the problem)