Most nocturnal mountain bikers, in fact, use two lights. One goes on the handlebar for steady, straight-ahead light. The other goes on the helmet and aims where the rider looks. Generally, about ten watts of output from each is adequate. Probably the most bang for the buck here is to get two Performance ViewPoint Pro Single lights, one with helmet mount and one with handlebar mount. These are good-quality ten-watt lamps, with about two hours of run time, and are currently on sale for $109 each. So, $218 in totalnot cheap but not outrageous either. If you get just one, get the helmet-mount model. Nightrider's Road Rat ($65) is a decent-quality ten-watt handelbar light that could be used in conjunction with a helmet-mount lamp.
The other strategy is to get a single light that is powerful enough to light the way by itself. At the top end are the new-generation high-intensity lights such as Light & Motion's ARC Cabeza. It uses a special bulb and high-tech reflectors to put out as much light as a 30-watt halogen bulb, while drawing no more juice than a 12-watter. Result: Four hours of run time. I've been using one on night rides the past few months, and it puts out more light than the combined four lamps of my two riding partners. Drawback: Cost. The Cabeza runs at $400, but it is a helluva light cannon. More moderately priced are lights such as Niterider's Digital Evolution, an extremely capable 15-watt light that sells for about $210. It's available in handlebar or helmet mounts; a very nice light.
Hope that helps!