UPDATED: See post #26 for pictures
I recently upgraded to an HID conversion kit inside a set of Halo projectors and was really unhappy with the results. I really wanted to upgrade to HID and the only way to really get the results is to do a full retrofit with HID projectors and d2s bulbs.
I took apart the Halo projector headlamp and removed the halogen projector. the halogen projector had a lens diameter of 2.5", so the only HID projector that's widely available are the E46s ( typically found in BMW and Audi TT ).