Phew! There is a lot of tackle in this thread.
Right, so, first off, on the subject of decibels, one must keep in mind that the scale is logarithmic. Think of it kind of like a Richter scale for earthquake, but far smaller in magnitude; a notch up is not a set amount higher, but a whole level higher. As an example, people commonly take 30 db to be about the level of a whisper, while 60 db is more of the loudness of a "normal conversation." Moving up in increments of thirty, 90 db can be roughly compared to say, a lawnmower, and 120 db is in the ballpark of a really loud rock concert and the threshold of starting to feel pain. The scale is small, but you can see how it steadily increases: doubling the decibels of a normal conversation is definitely more than doubling the actual loudness you will experience. (Behold! The troubles of trying to perceive an abstract model of reality, as it was created by man.)
Now, there are a lot of rules of thumbs when dealing with decibels. For this case, one worth noting is that when you have two like decibels, you can add two to their value to simulate what is being perceived. In other words, if you have two 30 db fans, it's a safe assumption to say you are perceiving 32 db. For mixed values (just to pull numbers: 5 db, 15 db, and 30 db) I think it might be safe to assume that the max decibel value is still the deciding factor - but I don't know for sure.
In the case of the Cougar fans, assuming 19 db (although the newegg link says 17.7), I think you'd work out the math to be 23 db. (Four fans at 19 db, if paired give you 21 db per pair, and you have two pairs of 21 db, making 23 db.) I think this might be bending the rule of thumb a bit much though, and I am going to say that it is just conjecture on my end.
On the subject of bearings, standard sleeve bearings can wear faster and go bad if mounted horizontally; all the lubricant that should be spread through-out the bearing can pool to one side and cause the whole bearing to wear faster. This is why most companies have updated with rifle bearings, which are modified sleeve bearings that have grooves to circulate the lubricant, or, in high-tier cases, fluid-dynamic bearings, which are sleeve bearings that have the grooves structured as such to have the shaft "hydro-plane" on the lubricant. Magnetic bearings operate off of the same concept, in a way.
I am not going to cover old ground with the ways of fan controlling, but, I will recommend Arctic Cooling F12 PWM fans. (
http://www.newegg.com/Product/Product.aspx?Item=N82E16835186033 ) They are very quiet fans at an affordable price-point, that have the ability to be "daisy chained" built into their design. In other words, without any splicing or hubs, you can connect multiple fans together to work off from only one 4-pin header on your motherboard. (If you don't have another 4-pin header, use a 3-pin header and Speedfan to automatically control their speeds.) If $10 per fan is a bit much for you, just remember that you don't have buy them all at once, you can buy, say, two for starters, and add onto the chain as you go.
Because I managed to stumble upon them:
http://www.newegg.com/Product/Product.aspx?Item=N82E16835233104 these should have a rifle-type bearing. If you get them, you're still stuck with the conundrum of how to get four 3-pin fans hooked up and going, but, still, I thought I would link them.