Here's a link to a recent thread with the same basic question you asked:

http://www.bikeforums.net/showthread...=#post11799976

There are numerous threads on the topic with a little research on back pages. Look them over and if you see something you're interested in, you can find more topics or ask questions on specific lights. Also, I hate to take you off these forums, but some of the best night riding and lighting discussions occur here:

http://forums.mtbr.com/forumdisplay.php?f=165