Your computers motherboard is the large circuit board which all other components connect through, it is responsible for handling communication between all internal components.
Like CPUs, motherboards are constantly evolving, a new CPU release often means a newer / faster motherboard is also released to support the new processors.
For Intel processors the top performing motherboard chipset is the Z series whereas for AMD CPU's the X series is the top option.
We change the motherboard used in a computer build automatically based on your selected graphics / CPU selection. This is due to different motherboards having different options on them for hardware support.
The motherboard upgrade cost is included with the graphics card or CPU upgrade price.