I would suggest that you maybe don't want to do that. Humidity will be a real problem, vermin will find the way into your home unless you pack the vent hole with copper or steel wool (which will impede your airflow), and generally come with a bunch of unanticipated consequences.
But worst is this: Your house has a complicated airflow/pressure balance situation just like your PC case. Anytime a combusting device turns on (furnace, hot water for ex), the entire air flow and pressure balance changes. A furnace or hot water heater running gas or oil will consume very large volumes of air. If you punch an unvalved hole in an outside wall without careful air balance and pressure testing before and after, then when the furnace kicks on it may draw huge volumes of outside air through that hole...and right into your PC. That outside air will be filled with dust, pollen, grit, spores...all settling nicely onto your expensive and sensitive electronics.
Not to mention the difficulty of making such a hole weathertight. If not installed carefully, flashed and caulked just so, you risk allowing water to get into the wall itself, which will guarantee destruction of that wall from the inside out. And that will NOT be a small repair.
So, all in all, I'd personally suggest you reconsider. I just don't think there is enough upside to make the project worth it. Your PC can not substantively benefit, IMO.
Best of Luck!