Lower "k" bulbs are brighter, so if it's purely improved lighting you're after then 4300k is the brightest, fact. 6000k, 8000k, etc are NOT as bright.
HIDs direct do a 50w kit with a choice of bulbs, and it's superb - about 3 times the lumens (light) output of a regular 35w halogen bulb. Got a 50w 4300k kit fitted in my Z4 and it's a major improvement, no pics though.
HIDs ain't legal though unless you retrofit the self-levelling components and the washer jets, but most MOT places don't bother as long as the colour's not taking the piss.