Just so we're clear here. What we're talking about doing is illegal from everything I've read...
http://www.danielsternlighting.com/tech/bulbs/Hid/conversions/conversions.html

(Daniel Stern Lighting is a pretty reputable source of info from everything I've read about the site on various forums.)

That said, I'm still intrigued albeit my interest is definitely tempered by the cost of replacement bulbs. For my Audi they're $75 each <img src="/forums/images/graemlins/zombie.gif" alt="" /> (albeit I can find them for much less online).