Mobos do take power, they don't just connect stuff. The chipset takes power as well as any controllers for usb, sata, audio, etc. It doesn't take much but even hdds that are less than 10w are taken into account. It's usually around 50w for the mobo.
750w is more than enough. Depending on other components, you could do with just a 500w. The mobo doesn't have a max wattage pertaining to gpus as the majority of the power comes from the pcie power cables from the psu to the gpu.