I would suggest getting a low power PC + picoPSU. That should draw about 40W, far lesser than a netbook while giving the expansion options not present in one. Plus the cost too would be on the lower end.
For the time being, I would not recommend Gigabyte C1037U board - great board minus USB3, which can be overlooked - an IC on the board got burnt, and Accel Frontline is saying they will repair only, not replace. The IC in question is the PCIe to PCI bridge chip.
The C1037 board comes with a dual core Ivy bridge mobile Celeron 1037U chip, soldered to the board, along with Intel NM70 chipset, which is a cut down variant of the HM75 chipset. This chipset lacks USB3 support natively. Plus, the Intel SATA controller does not support port multipliers. These are the two main cons. The pros are 17W CPU +4.4W chipset TDP power consumption, which is great. You get 3 SATA + 1 eSATA, which is more than enough.
Assuming you are a student, you could get a copy of Server 2012, and enable the Essentials role, giving you the ability to do backups of each Windows machine in your house.
The cost is 5.5K only for the board. RAM + HDDs + etc are extras. So, it may be expensive now, but a very useful device in the future.
From the ARM perspective, you could go with a GoFlexHome or Pogoplug. These will draw less power (12W all incl) and only allow downloading capabilities.