We tested Sapphire's Pure Mini with a low power Athlon X2 255 and, as you can see, the difference in performance between running the CPU in this and a full size AMD 790 equipped motherboard is negligible.
What's more, the power usage is much lower too.
You can also bung a decent GPU in there thanks to the PCI-E lane so a mini gaming rig isn't beyond the realms of possibility.
Video encoding performance
3D rendering performance
Gaming performance (800x600, v.low graphics settings)