Just make sure they keep things cool. We installed some quiet fans in a prototype system we were working on and they diddn\'t cool things off as well as the noisier ones. This system had a temperature gage in the software(industrial computer) and the difference was noticable. This probably won\'t be a problem for most computers though. Just keep an eye out for instability. When the processor overheats, it will crash or shut down. If you notice this when using certain fans, experiment with different ones.
my experience in silencing software-based studios say it is better to mount everything up to the noisy interfaces and little, noisy sampler-harddiscs into a massive silent rack anf forget all those expensive mini-solutions. You will see-If you silence the first drive, you will hear a new noise from another device previously masked. Then you silence the next one and so on. You can spend more 100´s or even 1000´s of dollars to sep by step silencing the machines. And then you have a single hard drive or Interface in your setup with a little, noisy Fan...
Take a silent Rack. It costs about $ 1000,o.k, but the solution works very well and you really get a quite setup!