Hella and Philips make replacement bults of stock wattage that produce 30% more light output using xenon gas. They are cheap and don't cause any electrical problems. I've been running the Philips bulbs for a year without difficulty and the light is noticeably whiter than other cars (but not that blue color of xenon lights).
Check out http://www.pgperformance.com.
"Lead, follow, or better yet, get out of the way!"E500 owners motto