nukemaster is right about the PWR_FAN mobo header - its intended use is only as an input from SOME PSU's that have a signal line to feed the PSU's internal fan speed back to the mobo for monitoring - it does NOT control this fan's speed. However, on some mobo's the header also does have 12 VDC on pins 1 and 2 to provide full power (no control) to a standard 3-pin fan. The only advantage of this (instead of connecting the fan to a Molex 4-pin supply) is that you get to see the fan's speed, but it is mislabeled as your PSU fan.
Both of the SYS_FAN headers probably can provide speed-controlled operation of case fans, but they are designed differently. A 3-pin fan is controlled by varying the +12 VDC line's voltage (pin 2). A 4-pin fan's header supplies +12VDC all the time on pin 2, and then the PWM signal on pin 4. A special circuit within the fan case uses that PWM signal to control just how much of the power from the +12 VDC line is actually fed through the motor, thus achieving fan speed control. So if you connect a 3-pin fan to this header, it always runs at full speed because the pin 2 line is always at 12 VDC. Well .... almost always. SOME mobo 4-pin fan headers are able to detect which fan type is connected to a 4-pin header, and will revert to behaving just like a 3-pin header is that is the fan you connect. Your mobo actually can do this for the 4-pin CPU_FAN header (see manual p. 46 under "CPU Smart Fan"), but it is not clear whether the 4-pin SYS_FAN header can do this, also.
Exact details of how each of the two SYS_FAN headers can control case fan speeds are not clear in the manual, but may be clearer if you install and run the application called EasyTune5 on the CD that came with your mobo. It allows you to tweak a number of items on the mobo, likely including all controls for the case fans on the SYS_FAN pinouts. Assuming that both can actually do case fan speed control, you could follow nukemaster's advice and connect UP TO 2 identical fans to each SYS_FAN output. BUT you would have to get 3-pin fan(s) to the 3-pin pinout near the back edge, and 4-pin fan(s) to connect to the 4-pin one near the front. In each case, connect identical lines in parallel together EXCEPT do NOT connect to the mobo any more than ONE fan speed signal line (pin 3) to each pinout. Leave other fan speed lines unconnected.