both of our theorys mayb rong
Please don't start a discussion about the laws of physics. That's a WAY-too-long topic which will result in many heads being lopped off.
but a pc case is anything but air tight
True, you should have a case that is modestly closed, so that the fans will be able to maintain the needed pressure. However, the amount of "seepage" through the cracks and joints of a case is relatively insignificant compared to the amount of air you are moving in and out with the fans.
There are two concepts to be considered when thinking about fan cooling in the box.
2. Relative pressure
The relative-pressure issue is easy to deal with. Basically, if your are creating dynamic pockets of pressure, or your fans are trying to blow your case up like a balloon, you aren't accomplishing anything (at least cooling wise). You want to maintain some kind of positive airflow in the case. (That means the air is coming out, not in. We want to get rid of the hot air, not keep it inside.)
Airflow can be a tricky subject, especially in odd-shaped cases. The general form is to have a fan in the front, sucking cooler air in, while having a fan in the back of the case, sucking the hot air out.
1. You want to make sure that you aren't creating a pointless loop with your airflow. The classic example of this is when you see a case with a front and back fan that are mounted eight inches below where the processor sits, basically passing air over nothing as they work. You want to design your setup so that the airflow pushed by your fans, draws the cool air over the components you want to cool.
2. Avoid deadspots or heat columns. Deadspots are places where the fans never are able to reach. For instance, if you have a huge wad of those flat ribbon cables and a bunch of devices, chances are you've created a barrier against easy, direct airflow. This will create a "deadspot" inside the case, where you essentially are getting no cooling or exchange of air. Here, the air will just heat up continually, and that can cause problems, especially
if you have a heat-sensitive component in the deadspot, such as a hard-drive or CD-ROM.
A heat-column is just what it sounds like. An example would be the heat rising off your hot video card. Usually, processors end up right over the video card, which puts them right in the path of the video-card's heat column. So basically, your processor isn't cooking itself...it's getting help. You want to make sure that this heat from your video-card is addressed. One good way is to make sure there's a fan blowing the hot air from the heat-column away from the processor.
3. Pressure. It's often overlooked, but I've seen setups with a ton of fans in the back, and one or no fans in the front. If your fans don't have the air to move, they won't really be moving. It's always good to create a positive airflow in your system. Make the front fans do the work of moving the heat to the back fans. Let the back fans do the work of kicking the hot air out of the system. If your back fans are "sucking" on the system, they aren't as efficient.
At the same time, if you have a ton of fans in the front, and only one or two in the back, you're kinda doing the same thing. Basically, you're cramming a ton of air into the box, with a very slow way of getting it out.
The bottom line here is
that you want to create maximum airflow, and maximum coverage. Your fans should work together, not against each other. Try to balance your airflow, between intake and exhaust. Avoid leaving heat-columns unattended. Avoid creating deadspots or redundant airflow cycles.
As for your slot fan, I would rather see the use of a case-side fan. The airflow works better and they cool much more easily.