Hi, I'm building up my own pc and this is my first time to do so. My motherboard is Gigabyte GA-MA785GM-US2H. So I have connected everything apart from the socket for SYS_FAN and NB_FAN. I actually forgot to buy the case fan(the one below PSU), so now I'm now confused to buy which kind of fan for SYS_FAN and NB_FAN. Is NB_FAN actually necessary ? My SYS_FAN has 4 pins and for NB_FAN it has 3 pins.
No. Neither is necessary. You could plug your fans into your PSU if you wanted too.
However if you want the benefits of board fan control then you will need to use the board headers.
The NB_FAN header is intended for the motherboard chipset. Its largely a leftover from the days of motherboard chipset fans which has largely passed us by. But its there if you need it. And you can use it for any fan.
Agreed, most northbridges now don't have fans, although with a water cooling setup one is usually added since there is no heatsink fan to cool the motherboard parts. So that's why gigabyte provided a NB fan header.
Its not neccessary to add a NB fan, or to get a 3pin fan for your exhaust but speed monitoring is a nice feature.
Likewise the sysfan is a 4 pin to allow a PWM southbridge fan that is speed controlled by the bios similar to CPU fans. It should also take a 3 pin fan as well.