I find a good, focused-beam 2.4 or 3.0 watt dynamo lamp w/ a halogen bulb sufficient for all my night cycling needs, no batteried ever required, and today's systems include standlights so your lights stay on even when stopped. That and a lot of reflectors and retro-reflective tape. I get my lighting systems online from Peter White Cycles, and my reflective tape online from Identi-tape. I get bulbs for older Union dynamo lamps from Reflectalite. All three companies offer superior service and delivery times.

http://www.peterwhitecycles.com/lightingsystems.htm
http://www.identi-tape.com/safetyonetwo.htm
http://www.identi-tape.com/hi-intensity.htm
http://www.reflectalite.com/halogenpage.html

To paraphrase an old Frank Zappa line from 200 Motels: "more than a mouthful is wasted". 3.0 watts is a mouthful; beyond that, you're falling victim to the great American tradition of "bigger is better", IMO.