Replace the headlights in pairs.
I have the Sylvania Xtravisions in mine, and like them quite a bit. They are inexpensive, but have good brightness and color.
The HIR lights are supposed to be crazy-bright...but, you have to be careful, because if improperly aimed, you could blind oncoming drivers.
You could, theoretically, get a ticket for them exceeding 1000L.
HIR are also very expensive...but, I believe they last longer than the normal lights.
The 9006XV for my car is rated to 850hrs.
I forgot to mention that the GE Nighthawk and Philips Vision Plus are well-regarded as well.
Here is a link to a letter from Sylvania regarding the differences between various bulbs (scroll to the very last post in the thread).
http://www.canadiandriver.com/forum/...,41744.20.html
I agree that the overall pattern of the Extravisions are pretty nice. It isn't like you go "WOW! THAT IS AMAZING!", but they do seem a bit better than stockers.
The price is definitely right.