I've got a few systems running on Sandisk Ultra USB sticks - specifically including the 32GB Phil linked, but mostly on the 128GB version of that one (Walmart had them on sale a while back and I stocked up).

 Plenty fast for a mining system, though not as fast as a good HD and nowhere near as fast as a SSD.
 
 For a pure mining system, a low-wattage Celeron or G-series Pentium should be plenty.