It's not a simple question
Like most things, the issue of fans and case cooling is more complicated than it seems. Here are the main points:
1. Keeping a case cool requires getting cool air into the case and hot air out. To accomplish this a case may have both input fans (cool air in) and output fans (hot air out.) Whether or not a particular case has both input and output fans depends primarily on the case design, but in most cases you can add whichever kind you want.
2. Fans themselves come in 2 basic varieties: fixed speed and variable speed. In general variable speed is better because the fan speed can change based on the current CPU/heat load. But variable speed fans are more expensive.
3. Fixed speed fans run at a constant speed/noise level. Their only input is power which they get either from a small 3-prong motherboard connector or a 4-prong Molex power connector coming off the PC power supply.
4. Variable speed fans have both power and speed control inputs and run at varying speed/noise levels. Their input usually comes from a 4-pin motherboard connector which supplies both power (3 pins) and speed control (4th pin.) These fans are called PWM fans because they use Pulse Width Modulation to change speeds.
Some variable speed fans come with their own speed controllers which is a small board mounted inside or outside the PC with knobs on it that you can turn to change fan speeds. But my sense is it is better to let the system itself determine the best fan speed.
4. In general cooling fans have almost negligible power requirements so there is really no need to be concerned about volts or watts.
5. Fans come in a variety of different standard sizes so it's easy to replace most any fan with a different one of the same size.
As an example, my case has one input fan on the front, 2 output fans on the back, and one output fan on the top. As delivered, all fans were fixed speed (to keep the overall case cost down.) It seemed to me that the front fan was a bit louder than I liked so I replaced it with a variable speed fan that plugged into one of 3 4-pin connectors on my system motherboard. Now the fan closest to me is much quieter most of the time, but does speed up when the CPU is working hard.
