MiniKAB 2: Intel Pentium G3258 (Socket LGA1150 3.2GHz+, Pentium Anniversary, overclockable) based MicroATX systems:
Very capable CPUs for MAME. Handles a lot of more demanding 2D or polygon-based 3D games very well (e.g. newer CAVE shooters, Tekken 3, etc).
Comes with a 64GB USB stick, and boots from this using Linux (Windows 7 Pro 64bit version also available, but requires additional SSD).