Also worth considering is Asus's line of EeeBox PCs.
For example, the EB1012P (dual-core Atom) or EB1021 (dual core AMD E-450). Both are small (less than 1 litre volume), low-energy (less than 25W), and have 6 USB ports. The 'barebones' versions (no HD, no OS ... you provide your own) retail under $190; fully loaded versions are about double that.
These are overkill for an FPGA host, but have the advantage of decent-enough performance that they can actually be used for 'useful' work while mining. Here is a 1021 to get an idea: