4200 is about where you want to be. It's the brightest. But PLEASE set it up right, if you're getting HID's. So many re****ed plug and play kits that blind others on the road. I want to take a hammer to them everytime. Also remember that HID bulbs are much more expensive than OEM bulbs.
But the cheapest solution would be to just add external aux. lights. Very cheap and easy to put on.