Just get a matching Xenon replacement bulb for your halogen bulb, in the 5000K color range, and do a straight swap. That's what I did and it's pretty much indistinguishable from the HID's. You'll save the price and hassle of changing the fogs over to HID's and will still get the look you want.

This is an example of what I'm talking about, but there are lots of different brands and qualities to choose from if you look around.
http://www.autoaccessorystore.com/ho...=detail&id=925