People on these forums have actually tried cooling an entire computer with a HUGE 25cm fan. In the end, multiple fans were always the result
actually,that's not strictly true,Felger Carbon & Bluefront both cool their PCs w/ just one 25/22cm fan IIRC.
The problem with mini-ITX is that most big heatsinks (which you will need for /semi-/passive cooling) don't fit. microATX is not that much bigger and fits all standard heatsinks.
I'm not too sure on thermodynamics heat transfer stuff, oddly enough since I'm taking an exam on it tomorrow
There's actually an equation to work out how much CFM you need, if you know how much watts your equipment puts out and the temp rise across the system, I'm sure it's in your textbook somewhere.