Well, you're getting pretty good advice here as it is.
In the competition for "fastest, coolest and most quiet", there are several dimensions to cool and quiet, one of them being "manual fan control", and another one being "automatic/programmable fan control."
For simplicity, and in observance of the "manual" possibility, the Sunbeam Rheobus ROCKS. It is supposedly capable of handling any string of fans (connected in parallel) drawing up to 24W on a single channel -- provided, of course, that you can supply the needed wattage for all such channels from your PSU. It is a solid, well-crafted unit, elegant in its simple design, made of aluminum for a 5.25" drive-bay, with precision unsplined aluminum rheostat knobs and an unpretentious LED light for each channel which changes from red to blue at a certain speed threshold.
The other unit I am currently using is the Aerogate II. It is just wonderful for monitoring speeds and temperatures for up to four fans or fan-arrays and four thermal sensors. I THINK it offers up to 18W of power to each channel, but you had better verify that. The master push-buttons and knob which controls fan speeds between channels and selects those channels for monitoring has something to be desired. It never fails but that an adjustment of one channel affects the equilibrium speed of another one.
One not-too-expensive choice, with high praise from a friend who opted to buy and use it, is the Silverstone Eudemon. The Eudemon is "programmable". I think it allows you to select a temperature threshold for spinning up a particular fan channel. But according to a letter I received from Silverstone tech-support, each channel only provides up to 14W of power.
Another class of latest-generation controllers are totally automatic, or provide the option to be totally automatic, programmable, and controllable through software under the OS. These add a hardware layer that communicates with the CPU through USB 2.0. Two entries are priced around $60: the VLSystem Zephyrus, and the mCubed T-Balancer. Both of these have bugs in their "version 1.0" software, although reviewers have high hopes of revision improvements.
Finally, in that class of USB-connected controllers is this one:
Innovatek Fan O Matic Pro
I am not completely clear as to the power capacity per channel. Reviews seem to be good -- I think I stumbled across one at THG dated July, '04. It still communicates with the system via USB 2.0. It would not appear that electronic components used to build this item are any more expensive than the other two models, but it is priced at $274 at FrozenCPU. One reason for the price may be the independent microprocessor on the circuit board that regulates the fans; another reason might be a surge in demand for a product whose earlier revision last summer was only around $130 -- in a market where the price of things usually goes down over time (as opposed to "up"); a third reason may be that the software is relatively bug-free and constitutes such an expensive development effort that it is being recouped in the item price; and the fourth reason may be that it must be imported from Germany.
Take your pick. Save your pennies.
In the meantime, I will avoid the extra hardware layer, control my CPU fan from the motherboard with Alfredo Comparetti's "SpeedFan", and send Alfredo $10 via PayPal out of sheer gratitude and without hesitation.