I bought a pre-build SamFlex with (overclocked) 800MHz. As far as I understand the chips are designed to work fanless under rather rough conditions, so overclocking them when you ad a fan to the box should be no problem. (I am a software guy - my oppinion on HW is hearsay.)
My box runs quite stable, although - well, let´s say Gurus/Reapers still are part of the user experience. Also my USB-Stack/driver seems to be easily disturbed, but USB is bitchy sometimes even on a mac, so I guess I can cope with it.