depends on the application, if you are looking for HID for your headlights, follow SoCalXTerry's recommendation and do a retrofit. It is easier than it sounds and the output is a million times better than just throwing some HID conversion bulbs into your stock halogen housings and blinding everyone with glare(dont listen to people who tell you to just aim your lights down to reduce glare, that doesnt reduce the glare, it only limits how far you can see). I purchased my retrofit at theretrofitsource.com and with a discount code on thenewx, you can get 20% off iirc, which makes the retrofit very reasonable.
if you are looking for offroad lights, follow LupinOne's links and use the HID conversion bulbs in the stock housings, due to the limited space a retrofit is very difficult with the stock off road lights, and since the offroad lights are used offroad, the glare is a non issue. i run conversion bulbs in my offroad lights and am happy with the output.