My upright uses two fans blowing IN from the sides (two b/c they are a design element) to the PC compartment, and one at the very top of the cab blowing OUT. To maintain air flow bottom to top, I put vent holes in the monitor shelf b/w the PC compartment and the monitor compartment. Works well for me.
My cocktail, where the PC lives just under the table surface, has two passive vents in the bottom of the cocktail carcass, and two fans exhausting OUT that pull hot air right off the top of the PC's M.B. In this case I used two fans to get good airflow because the M.B. and the monitor coexist in a very compact space.
All of them run off external power supplies. A spare AT PSU in the upright, a spare 12V wall wart in the cocktail.