I was just reading up on the OSRAM/Sylvania Silverstar bulbs (Xenon gas charged bulbs). It's the same company, but the OSRAM (euro) version of the bulb puts out 50% more lumens than a standard halogen bulb, with a slightly whiter light, while the Sylvania version puts out about 30% more output than a standard halogen, but is significantly whiter and closer to daylight color due to a barely noticable blue tint added to the glass (which is also responsible for the lower lumen output). I use the Sylvanias and love them. There is a significant improvement in lighting performance compared to the halogens due to the higher lumen output combined with the whiter color (the closer to daylight color, 4000-4300K rating, the better the road is illuminated).

Stay away from anything that claims to be rated at over 4300K, they will be garbage. They will have a heavy blue coating on the glass to achieve a faux-HID look, but the bluish colour makes it very difficult to see at night in addition to drastically lowering the lumen output of the bulb. I tried a pair once that had a 5500K rating, but were 100W. I figured the higher wattage would make up for the loss of illumination caused by the bluer light, but I was wrong. The visibility was so bad I pulled them and tossed them in the garbage after my first drive.