Do RESEARCH! Yes, carefully study up on the lights before deciding on one.
Most people will tell you that 4300K bulbs are the way to go, and I agree. Choosing any color greater than 4300K will add a blueish tone to the lightsource.
But this is not always the case....
Different ballasts will effect the final tone the light emits. I have seen some perfect (and expensive) 4300K bulbs look totally blue because of really cheap ballasts. These bulbs require AC current to operate, and if the AC frequency is not dead on it will change the hue of the light output.
On another note, I have friends who are just pissed that the blue 8000K bulbs they bought look totally white (once again, frequency of the ballast too high). It will also lead to a short bulb life.
Mine turned into almost pure white with a little hint of blue. Not enough to really tell much of a difference, but they seem just about perfect for me. I put my 8000K lights up against another car last night who purchased 400+ dollar ballasts and his were VERY blue compared to mine.
But to be safe get some decent ballasts, 4300K bulbs, and go for it! You can always purchase a different bulb later and try one out to compare if you are thinking of moving up into the blue range.
Here are some links for the HID companies I recommend...
The ones I bought, I went cheap but they are digital...
http://www.xenongate.com/index.php?cPath=37
Some expensive but very well designed kits...
http://www.xtralights.com/index.asp?...it+pair&Page=1
The last site also has a nice mouseover picture on the main page showing the various differences in blub color from the driver's view. The blue gives great contrast compared to standard white, but once again you really need to think about how blue you want your bulbs to be. Just click on the HID Kits for Cars link for the demo.