This all depends on personal preference. I've rode with a Fenix L2D Q5 (180 lumens) and found it completely inadequate. I use 2 dealextreme P7 flashlights, somewhere around 800-1200 lumens combined.

Sometimes I'll run one on medium, sometimes the other, sometimes both on high...if one dies I'm not caught in the dark.

Two 160 lumens might do it. Give it a try.

EDIT (July 11, 2009): I discovered my L2D LED has been damaged for some time now, so my Fenix opinions are not accurate.