Just bought a Gigabyte mobo and an Antec Case. The case has 2 fans which connect to the power supply. But coming out of the power supply is a a black and blue wire called the "3-pin fan sensor" The mobo has "pwr_fan" and "sys_fan" connectors. Should I connect the "3 pin fan sensor to one of those, even thought they are already connected to the power supply. I am guessing that mobo has those sockets for case fans which don't connect to the Power supply?? Thanks in advance.

the mobo's connectors are for case fans. Usually you plug your fans there, unless you have some place else to plug them (psu, or in my case, the front panel). i don't know about your psu's fan connection though...perhaps it's there to give your mobo the ability to check the psu's fan's speed? how do your fans connect to the psu? I think it's better if you connect the case fans to the mobo...that way, the mobo can sense their speed, etc.

I wouldn't connect all the fans to the mobo yet. Check the power of the fans being used. A lot of the newer case fans require so much wattage that they should be connected directly to the PSU so they get enough wattage. Otherwise, there is a risk of burning out the tracks on the motherboard. If changing/monitoring case fan speed isn't an absolute necessity, and you have the 4 pin connectors, hook them up to the PSU and lessen the risk.