The Cubieboard is probably the best solution for NAS, cheap (50$), open hardware, open software, SATA connector but still some closed part in firmware, as on every SoC. Use Allwinner A10, that is not the more powerfull processor (1xcortexA9 @1Ghz, but good enough to manage fs, and even 1 core Mali400 for the 3D and CedarX for really powerfull video hardware decoding (quadHD). You can alos findi this SoC in Melee2000 that have 1GB of RAM too and is a dedicated videobox.
Originally Posted by kgardas
Else if you need power, the best solution, is probably to use the first CortexA15 out, the Samsung Exynos 5250, that is on a more expansive board (~250$), this board also have SATA port and Samsung is involved in Linux kernel développement for their hardware support, that's another good point.