You can use it, yes. However, it can only be used after you click on Cortana/search and then the microphone. The 640XL is not capable of the active listening required to wake the phone like the 1520 is.
NO. Lumia 640 does not sport the hardware thats needed for active listening... however you could press n hold the search button to activate cortana or use a bluetooth or wired head set to activate her while driving or riding a bike