You need one ballast for each bulb, so if you wanted to have HID's in both the high and low beams you'd need four ballasts. You can buy H1 HID bulbs and use them with the ballasts you already have for your 9007 bulbs.

HID's really aren't the best option for the high beams, as they don't like to be flashed on and off quickly. I'd recommend replacing the 9005 high beams with 9011 HIR (halogen infrared refraction) bulbs, which are significantly brighter than standard halogens.

Information on HIR bulbs:
HIR Headlights 9011 9012 bulbs