For a robust setup I tend to go with servers when buying second hand, things like ECC memory and power supplies designed to be on 24/7 add up to a much, much more stable system.
For a cheap reliable new system, these are really hard to go past.
http://www.shoppingexpress.com.au/bu...371/658553-371
Less than $300, use ECC RAM, 7 USB ports and have 4 drive bays and are tiny and really reliable.
Not a speed demon when it comes to performance, but are still pretty quick, ultra reliable, tiny and use almost no power at all.
There are a few different flavours of the HP Microserver, some are a bit more grunty. They also make a great home media server, can playback 3D Bluray without a hassle, and again, lots of room for storage and don't add much to the power bill.