And, that (if one picks some quiet fans) is, frankly, overkill.
Just get some good, quiet fans, and mount them to a solid surface (and, one that has good noise-dampening properties), and is itself solidly
mounted to the cabinet, and you'll be fine.
Anyone here remember me moving my Media Center into an HTPC case, and stuffing it into a rather smallish stand? This post
is where I started working on the cabinet cooling system. Three Scythe 120mm case fans; they're barely a whisper when they're running.
In fact, I can barely hear them when everything else (TV, receiver & speakers, and the PC itself) are off.
Aside from being, by their nature, quiet fans, they're also mounted (solid-mounted, mind you) to a good surface - the back panel of that cabinet is 1/8" Luan (basically, thin plywood - it's used in cabinet-making; typically used as a back panel, or a drawer bottom), and there are about 10 screws attaching it to the back of the cabinet (mind you, it's about an 18x18 opening.)
Again, can barely hear them with everything else off; if only the PC is on.... well, the PC (which, itself, is not loud) is louder.
Like others said here, the panel material helps a lot. Wood, by its nature, dampens vibrations. Luon is fairly cheap, easy to work with, and readily available (hell, you can get it at HD!)