Also, If you are planning HIDs, you will want to disable the DRLs. The DRLs work by adding resistance to the headlights (same bulb, just running at a lower voltage) With HIDs, this will mean the ballasts will be receiving low voltage any time the lights are off and just DRLs are on. This isnt good and can sometimes cause a strobing effect where the lights will flicker on and off.
Almost all cars will be more dim at idle as the alternator is not producing as much electricity. if you bump up the engine speed a few hundred RPM (which also happens when the A/C compressor kicks on as stated above) you should see an increase in voltage as well as slightly brighter headlights. In fact, at idle with headlights and the blower motor running on high, I can hear the fan speed change slightly when the turn signal is turned on and flashing. My voltage gauge shows about a half of a volt drop in these situations, even though it is still well within spec.