mrs fry, all the posters above give excellent advice, but I'll lob in my two cents anyway.
I ride a bit in heavy urban traffic, and also on towpaths and through parks, so I have a quite bright front light for the non-road stuff, and a really bright rear light so that I can be seen from the rear on the road. As people have said, it is a good idea to have a back-up light front and rear, in case you lose a light, for what ever reason...(eg, dud battery, or light falls off)..
Here is the stuff I use. I'm not saying it is optimal, but it is about the minimum I would be happy with.
Rear light. Cateye TL-LD 1100...£24
Front light. Hope Vision 1....£72
Back-up lights are some £15 light package from Argos, not great, but better than nothing.
I suggest you stick to one battery size, to keep things simple. I use AA rechargeable Eneloops, £20 for 12 batteries. They should be good for 1000 cycles, but the manufacturers are usually a bit optimistic.
The charger is a Technoline BL-700, about £28 from BatteryLogic.
Yes, that all adds up to a horrendous amount of money, but I just want to stay alive, and being unseen in traffic is very bad news indeed.
Ride safely.
Edit: You can probably use a much less expensive front light than the Hope Vision 1.