I wanted to post this conglomerate of information that I've accumulated over the years so that people can educate themselves better on the applications and uses of different types of automotive lighting.
Much of this has been copied and pasted and tweaked from the sources listed at the bottom.
1. Is 8000K brighter than 6000K?
No. The Kelvin scale is a measure of colour temperature, not intensity or amount of light. HID setups that produce maximum light output are around 4100-4300K colour temperature, and the higher you go, the less light output. A 12000K HID kit will be dimmer than your stock halogen lights. 4100-4300K kits will have the colour of all the HID lights you see on cars that come equipped with them. The color is very white, while light output becomes progressively bluer as colour temperature rises; 6000K is white/blue, 8000K is very blue, and above that you go towards purple. 4000-4300K is the optimum visible spectrum for light. Anything else, even if brighter, produces less light on the visible end of the spectrum.
2. What HID kit should I buy? What is a good price?
None. Don't do it. Aftermarket HID is sold by a large number of companies, but only made by a few. Here's an example of a bunch of kits made by the same Chinese factory: WebCite query result
ALL of those are the EXACT same kit, made by the same company.
If you're going to be doing an HID retrofit, the only option is an OEM kit in D1R, D2R, D1S, or D2S. Any other bulb is just one of those four, rebased.
OEM HID kits are the same products fitted to many vehicles (usually luxury) by manufacturers before they are sold as new vehicles. See this link: Carpassion.com - Europas große Sportwagen und Luxuswagen Community. Notice that almost all ballasts and bulbs used by car manufacturers are Bosch, Osram, Philips, or Hella. Bosch and Osram are not as readily available for retail, but Philips and Hella are. Kits that are not made by these companies are most likely not of the highest quality, although some aftermarket manufacturers such as Brightstar and McCullogh have been recognized as high quality products as well. Philips/Hella kits are generally considered the best.
However, projector retrofits aren't always the best idea. Although the cutoff is much nicer, it's about optics, not lighting. In many lamps, the cutoff will remain the same regardless of what light source is behind it. Halogen bulb, HID capsule, cigarette lighter, firefly, hold it up to the sun—whatever. That's because of the way a projector lamp works. The cutoff is simply the projected image of a piece of metal behind the lens. Where the optics come in is in distributing the light under the cutoff. This is why some E-Code headlights are suitable for both HID and halogen use. And, as with all other automotive lamps (and, in fact, all optical instruments), the optics are calculated based not just on where the light source is within the lamp (focal length) but also the specific photometric characteristics of the light source...which parts of it are brighter, which parts of it are darker, where the boundaries of the light source are, whether the boundaries are sharp or fuzzy, the shape of the light source, and so forth. A retrofit is a very difficult process that, unless done just right, can result in a LOT of hotspotting, even if the glare is significantly reduced.
3. Can I put HID into my stock halogen reflector lighting assembly?
Technically yes, but you shouldn't under any circumstances. Although it may not seem so, directing the light produced by a headlight bulb to the right places requires a good deal of precision. Reflector assemblies on cars that come equipped with halogen bulbs are made to direct the light produced by halogen bulbs correctly. Replacing those bulbs with HID bulbs that produce three times the light yields a terrible beam pattern. You will have light going all over the place, most importantly into the eyes of drivers of oncoming vehicles. Even if you can tolerate such a beam pattern, your lights would be a safety hazard on the road. Many people try to solve this problem by lowering their lights. However, to reduce glare to acceptable levels, you would have to lower HID lights in halogen reflectors so much that you wouldn't be able to see anything in front of you. The HID can also overload your factory wiring, and, in extreme cases, cause things to burn: http://forums.nasioc.com/forums/show....php?t=1660625
Next I'm going to copy-pasta an FAQ from my NASIOC stomping grounds that will explain the science behind all of this.