A good place for the HID bulbs is right here:

http://www.hidplaza.com/

The cheapest way to go is to get the H7 bulbs, but the car kit instead of the motorcycle one. Its 194.99 per headlight, and for the 600RR, you need two, almost being 400.00. Or you could just get the H7 Automotive kit (same thing) with two bulbs, transformers and ignitors for 309.99. They go from 3000k (yellowish) to 12000k (dark violet). They will give you the brightest possible light output.

I've used Silverstar bulbs in my car before, and they are a joke. I took them back right away. Didn't make a freaking difference. Never tried them in my bike though. I do know that the Silverstar turnsignal bulbs are sweet. They are mirrored bulbs in a blueish-green tint, but light up amber. 100% street legal too. I use those on my 600RR.