I went with these guys to replace both lights (XB12R) with HID:
http://www.hidfoglight.com/productsAndServices/retro/hidSlimHeadLightOrderPage.html. They sell a system specifically for bikes.
The bike kit comes complete with everything to replace both hi/low sides. It also includes the newer slimline ballasts that are only 2.5" x .5" x 3", so they're much easier to mount in tight spaces.
Installation is a breeze. No modification of the stock light housing is necessary. Replace the stock bulbs with the HID bulbs, mount the ballasts, and make a few connections. I mounted one ballast to the outside of the relay box, and the other to the outside of the fuse box. It looks clean like a factory install.
Once finished, my low beam is bright enough so that I hardly ever have to run the high beam. I went with the 6000k bulbs, which are bright white with just a tinge of blue. At 35 watts, the system runs much cooler than stock, draws less power, yet throws out much, much more light. $199 for a two-bulb setup is pretty competitive, too.
If you have any problems, the company is very responsive via email and/or phone and will exchange any part that doesn't work for 1 year.
