Motherboards do not have a maximum wattage. The equipment will draw the power it needs. Power supplies are most efficient when working in a 20-40% load profile.
The 24 pin connector is for the board, the USB connectors, fans, powering peripherals like CD/ROMs and hard drives and all the busses.
The 4 pin is for the CPU.
Only full tower cases or rack mount servers need an EATX.
You can only put one GPU on that card. With the 4100 and a cooling fan like a CM 212 and a good GPU you should only need a 550watt supply. Make sure its single rail, 80+ bronze minimum. Corsair, Antec or Seasonic. Some Rosewill supplies are good as well. If you get a mid range GPU like a 7750 that doesn't take external power, the 380watt should be able to limp along, but you will not have much head room.