I at least 2nd the Cygolite brand. I bought the cheapest Cygolite, the Metro that I admit is an odd light for most but not for me. The Metro has dual beams with 6.5watt flood and 6.5spot for a total of 13watts; it will run for 5 hours on 1 beam or 2.5 on both; here's the odd part, it uses 6 D batteries which can either be throwaways or rechargeables-I use both. I liked the idea of the 6 D bats because I was no longer tied to a wall and if by chance the bats when dead on a ride I could go into any store and buy more. now you may have to buy a recharger if you go with rechargeables but I already had one for the kids battery stuff. This light bright enough for me, but the cost of this unit was only $48. Cygolite also has a model called the Rover NiMh Xtra that also has dual beams with total watts of 16 that will run for up to 6 hours on one beam for about $78.
I would also recommend you buy a cheap bright amber or clear (Xenon or LED, Vistalite brand?) flasher that will go on the front to attach the attention of drivers eyes, because no matter how bright your light is due the much smaller headlight surface it just looks like a small flashlight further away then it really is or the drivers just don't see it all. The flasher will draw their eyes to your bike.
Make sure you also have a bright taillight like the Cateye LD600 or LD 1000; the 600 is less expensive ($18) then the 1000 ($35) and plenty bright enough, but mount the 600 vertically so it can be seen at least 180 degrees. The 1000 is the brightest taillight in the industry.
Barend lights are also very effective cost about $18 and should be used in conjunction with a taillight.
And don't forget reflective leg bands and jackets etc.
My whole lighting system cost me $100 which includes the Cygolite Metro, Vistalight Xenon flasher, bar end lights, and the Cateye 600; and I get many compliments about how well drivers can see me.