Case Fans: Speed Control
Option 1: Do It Yourself For Free
Your power supply already has 12 V and 5 V rails. As a result, you have the option of employing the difference between them (yielding 7 V) to drive a fan using three different voltages. As mentioned, fans that don't support the four-pin PWM connector can be speed-controlled by means of their supply voltage. The lower the voltage, the lower the fan speed. Lower fan speeds typically result in lower noise.
But be aware that every fan has a start-up voltage, required for the fan to start spinning. The start-up voltage is slightly higher than the minimum voltage for the lowest possible fan speed. For instance, some 12 V fans run at 5 V once they have started up. There's a chance they wouldn't fire up at 5 V, though. Thus, you have to experiment in order to figure out if your fans really start up at the voltage you want to use. In order to use all three possible voltages, you can build an adapter yourself using the following illustration:
You can buy a pin extractor tool to pull pins from Molex connectors. Once removed, they are easily reinserted at any position. The pins are held into place by two barbs. You can use tweezers to squeeze the barbs and then gently slide the pin out by tugging on its cable. The magic word here is gentle. Don’t rip the cable from the pin. Do not cut cables and reconnect them with duct tape; that's not professional and might even cause a short inside your gaming rig.
Option 2: The 7 V Adapter with Series Resistor
You can find fan cables with series resistors in PC stores. However, series resistors are matched to certain fan wattages. The higher the wattage, the more voltage drops at the resistor, at which point attaining 7 V for the fan is hit and miss. A powerful fan can even cause the resistor to burn out, so be careful not to exceed a fan power of 1.5 W or so.
Option 3: Aftermarket Fan Controller
PC shops seem to stock fan controllers by the bucketful. Before buying, check the maximum power per channel and perhaps the total peak power level. Do you want to control a PWM-based fan as well? Better make sure the controller supports it.
Whether you want to control your fans with old-fashioned knobs or digital touch panels is totally a matter of personal preference. Let your fashion sense and wallet guide you.