^^^ what he said.
A good light bar will have a flood and a spot section and will light up enough road that you'll be able to see anything well beyond your stopping distance - in other words, you won't have any problem stopping once you see something. We were towing up to Mudgee at Easter 2014 and ended up deciding that the 120W light bar was providing more than enough light.
It doesn't help in fog, however. As you might expect, the massive amount of widespread white light coming from a light bar turns fog into a solid white wall. HID lights might not do that - at the very least, if the reflectors of the HID lights were designed right, you'll have two very bright cones emanating from the front of your car but you'll be able to see past it. The combination is therefore quite worth it.