http://www.flightglobal.com/landingpage/apache.html (gathered from wikipedia)
The main difference : the "D" variant is housing a millimeter-wave Fire Control Radar target acquisition system as well a Radar Frequency Interferometer inside the Dome over the rotor...
So without the "Dome" it can't be called "longbow" or AH64-D.
The two models have "radar", but not the same and only the D variant is able to detect and attack targets even concealed by obstacles plus some features such as more powerful engines, radio modem for sharing tactical datas...
Hope it is clarifying things a bit...