I've just sorted out our three machine network (all in the same room) with a
Zalman solution.
Their 80mm case fans are seriously quiet and work well for a CPU fan if used in conjuction with a 60-80mm adapter. Beware though that although they come with a resistor (I think) that allows it to operate in silent mode I noticed that one of the machines CPUs heated up quite badly so I removed the resistor. To be honest though it's still pretty quiet and that's my server so it'll be hidden away in a cupboard.
I have the 60-80mm adapters on the two slower machines but for my beast I went for the Zalman CNPS7000B-Cu (Nice name huh!). It's a 90mm fan surrounded by copper fins. Just need to watch out for clearance between the fins and the PSU which BTW is the Zalman ZM400B-APS. I had to buy a new motherboard to enable me to even fit the CPU cooler onto the board (it needs four holes in the board around the CPU). It's still brushed up tight against some capacitors which is a little worrying but I think it'll be ok. They also do a 120mm version but I don't know how anyone gets THAT thing into their case! If you are considering getting one of these CPU coolers then checkout Zalmans website for compatibility with motherboards.
I'm in the process of an unattended install of four machines while I write this and while I've swapped out all the case fans, the CPU one is waiting to get fitted to the new motherboard. Once it's done I'll post my results.
Finally, Zalman also do heatsink only replacements for those noisy northbridge fans. So I got three of those to complete the solution.