honestly speaking, you need SATA/SSD for NAS and not just slow SDHC cards. Pity you don't have an option to give a try to cluster of for example free scale i.MX3 Quick Start Boards or new i.MX6. That may be better than pandas IMHO.
The Cubieboard is probably the best solution for NAS, cheap (50$), open hardware, open software, SATA connector but still some closed part in firmware, as on every SoC. Use Allwinner A10, that is not the more powerfull processor (1xcortexA9 @1Ghz, but good enough to manage fs, and even 1 core Mali400 for the 3D and CedarX for really powerfull video hardware decoding (quadHD). You can alos findi this SoC in Melee2000 that have 1GB of RAM too and is a dedicated videobox.
Else if you need power, the best solution, is probably to use the first CortexA15 out, the Samsung Exynos 5250, that is on a more expansive board (~250$), this board also have SATA port and Samsung is involved in Linux kernel développement for their hardware support, that's another good point.