I cannot stress how awesome www.hidplanet.com is.
I will also link this site as it is VERY informative.
http://faqlight.carpassion.info/
I’m retro fitting either RS6/A6 Bi-Xenon ECE Projectors or Porsche Bi-Xenon ECE Projectors into my JDM S14 kouki headlights. Leaning towards the RS6’s as they have more tolerable dimenions to work with.