I'm trying to start a big debate on this, as I proposed a Full setup for The Fridge (freezer cooling) I'm particularly fearing the condensation issue, but I've got some tricks that may help. (see The Fridge connection or how...)
For the outdoor air thing, you don't need to worry, since cold outdoor air is USUALLY not below it's Dew Point (the temperature at which condensation occur for a given relative humidity). And as the air gets into the ducts to your case, it gets a little warmer, which is lowering it's relative humidity. Anyway, since in this case your computer is getting cooled by the cold air itself, there's no chance that any surface becomes colder than it (the air, I mean). (especially not your CPU). The condensation would occur outside the PC 's case, which you would take care to seal a little bit. One duct brings the air from outdoor in the case, the second duct brings it back outdoor...
Nice n' Easy