I have a HP 5730 with a new pci-e sidecar that's on a closet shelf. I bought it a while back to experiment with. It works great. PM or email me if interested.
It has Athlon/Sempron 2.0+ Ghz cpu and 1 GB ram, IDE 1GB DOM module. Best IO I got was around 80-100MB/s over its GB nic. Not bad.
MY NAS is a E5300 cpu on a Intel DG45FG ITX board. Raid 0 pair + OS drive.