Home Theater Cabinet Cooling

Here's my dilemma:
* I just built my first HTPC
* I need to fit it into my media cabinet
* Physically, it will (hopefully) fit, but media cabinet is already really hot (qualitatively so, at least)

Right now I passively cool the cabinet by keeping the (solid wood) sliding door slightly opened. It has seemed sufficient (qualitatively so).

I've worried that when I add the HTPC, and before I remove my blu-ray and cable box (soon, oh very soon), it will overheat the cabinet.

I'm trying to figure out if adding a 120-140mm fan to the back of the cabinet will do the trick, but more importantly, where to place it. I can place it on the top back corner, opposite the door opening, and pull warm air out, thereby letting cool air in the door opening (Option A). OR, I can pull cool air in the bottom, letting the warm air out the opening (Option B). (Please see attached 3D image)

My question is: Is there a substantially better option, and if so, why? My preference is to pull cool air in so I don't blast dust on the newly painted wall behind the cabinet.

I took a thermodynamics and heat transfer college course, but for some reason can't apply what I learned. Or maybe I'm just lazy and figured someone here has done the research.  

Thanks in advance!
Bill