Personally I never plug any fan into the motherboard, those fan pins on the motherboard have a habbit of blowing (especially if you're using powerful fans.) You would be better off plugging the fans into a fan controller. If for some reason your cpu fan fails, once the cpu reaches an unsafe temp, it should automatically downclock itself and if it is still too hot it will shut down the system.
In terms of NB and SB volts they can both be set in the BIOS and in regards to to NB and SB temps am pretty sure the gigabyte system monitor software will show the NB temps, not sure about SB though...but ususally the SB doesn't get that hot