Fans 101:
Fans are obviously meant to push air to cool computer components. However, it is equally obvious that they do not always need to push the absolute maximum amount of air; often, only a small amount is required thanks to the "race to idle" strategy employed by computers.
So how do fans know when to be loud or quiet? It depends on the design, which is physically built into the fan. Some of them 3-pin connections, others have 4-pin connections. The key difference is that a 3-pin's speed must be manipulated through voltage, while a 4-pin, if plugged into a four pin slot on the motherboard, can be controlled through "Pulse Width Modulation" (PWM), which is much easier. This is because fans need a minimum voltage to function, so your range is a bit more limited on that side. In theory, a PWM fan is quieter overall since it makes less noise during the main state of computing--idle.
You can plug each fan into the other's slots, but you'll either be losing the PWM feature (no pin to support it) or not utilize the PWM possible (no receiver for the PWM on the fan).
Other important features of fans include the static pressure, noise level, and CFM rating. The first two are typically more important. Think of it like rating a river: static pressure is how forcefully the water moves, noise level is how loud it is, and CFM is the volume of water it moves. CFM doesn't matter as much simply because many fan waste most of it: the moving air spills everywhere and doesn't do a good job of moving heat. Silverstone AP121s, for example, have a low CFM rating but the air it does move is exceptionally well focused, allowing it to perform better than higher CFM, but less focused fans. Manufacturers exaggerate these specs, so get onto XBitLabs or a similarly reputable site to find the real statistics.
When installing fans, keep in mind that they have an airflow direction: it's labelled on the fan (one arrow pointing sideways to show the direction of rotation, and another arrow facing the direction air moves) roughly like this: ^ ->. Make sure your arrows point the right way.
In the actual case, it's important to maintain a cosistent direction of air from front to back and top to bottom. The reason for this pathway is the arrangement of parts. HDDs don't make a lot of heat, but are impacted by overly high temperatures, so they go next to the front intake fans. The GPU comes next (typically, as warm air rises and the GPU is positioned below thanks to the motherboard orientation), and is the warmest part, so it needs the coolest air. Finally, the air reaches the CPU on top--it's still warm, but not nearly as much as the GPU. Make sure you have no fans opposing this pathway, or they'll end up fighting each other, reducing cooling.
You can have a positive pressure case or a negative pressure case, depending on whether you're moving more air in or out--this is where CFM comes in. Negative pressure (more out than in). typically cools a bit better (by a degree or two) but ends up attracting a lot of dust--a negative pressure case is basically a weak vacuum. Use air filters if you do negative pressure. Positive pressure, more in than out, is a bit warmer but keeps the case cleaner. Either way, try to clean the computer out every few months--it helps keep the fans moving at the rating they should and helps the computer's longevity.
/101
If you don't like the color of Noctuas, look into the Phanteks series. The high end ones are identical in performance and appearance to the Noctuas, but have a better color scheme (they produce black, red, blue, and silver versions). Phanteks are typically around $90-95. On the other hand, any real computer geek will recognize Noctuas for what they are, and praise you for them.