I use a headlamp that's rated 70Lumens and it does the job pretty well. I like it because it shines the light in the direction that I want to see, not just straight and I can use it for other things not cycling related (hiking, working on cars, etc...). If you are going to be moving pretty fast, anything over 100 lumens should be fine. I have a 130 lumen flashlight that I can focus the beam on and it is plenty bright, you can see a good hundred feet in front of you.