They are that quiet, that when i shorted the black and green cables to turn it on to test (without it being connected to anything) i thought it wasn't working, i had to physically look at the fan to make sure it was on. Even on full load, i haven't seen the fan spin up any more, or at least not audibly. Fantastic PSU's imo.
If it's an HTPC though, i would assume you dont have a huge power requirement and the HX520w would most likely be enough for you. (will need specs to make sure though...)