I have been running the Sylvania Silverstar bulbs for almost two years and have been very happy with them.
I found out a couple weeks ago that Osram (Osram and Sylvania are, I guess, the same company) offers the Silverstar bulbs in Europe, and they are much brighter than the Sylvania's offered here in the US.
They are called Osram Silverstar +50, and they are supposed to be 50% brighter than (I guess) OEM bulbs. I have heard nothing but great things about them. You can find them on Ebay, of from this place...
KHY Motorsports
KHY Motorsports offers them for only $30 shipped. I have ordered a pair and will compare them to the Sylvania version as soon as they come in.