Try the AGT 35W one's. I have them on my Yamaha Raider and love them. You get a low beam HID (which is brighter than any halogen high beam) and your high beam stays halogen. THis way you have a bit of color change when you switch them from high to low which alerts drivers of your presence more. Also if the HID ballast were to fail you'd still have your Halogen high beam. They run about $120 for a pair but sometimes you can find someone selling them individually for $60. The mini ballast fits in My Raider's headlight housing, barely. I think it would be easier to fit in a stock xs650 headlight.
There are a few brands to avoid but I can't remember their names. I want to say its like DDR or something. Also avoid the 55w one's...to much for most bikes wiring systems.