What parts of a system dictate the maximum amount of memory it can use?

[Jskid]

What parts of a system dictate the maximum amount of memory it can use? Does it come down to the number of lines in the address bus?

 

[Buckshot420]

The motherboard dictates how much RAM a system can hold. The OS dictates how much RAM can be used.
http://msdn.microsoft.com/en-us/library/windows/desktop/aa366778(v=vs.85).aspx