Here is a link to everything you need to know about building very bright LED lights for use with your dynamo hub.
http://pilom.com/BicycleElectronics/...lectronics.htm
Here is a link for the brightest star mounted LED suitable for using with a dynamo and bridge rectifier.
http://www.dealextreme.com/details.dx/sku.2394
If you copy the single LED and elliptical optic on Martins site here is where you get it for use with the Cree LED.
http://www.luxeonstar.com/fraen-elli...leds-p-306.php
A single LED light will be much brighter than your halogen lights and you will not need any batteries.
Here is the updated circuit I use which includes a USB connector for powering and/or recharging USB devices like a cell phone or GPS unit.
The 4 Ni-MH batteries must be solder tab type with no possability of an open or poor connection which would allow unfiltered and unregulated power into the USB device possibility damaging it. My original circuit used batteries to filter and regulate a single 1-watt LED that was not rated for more than 350mA. The new power LED's can take 1,000mA so a standard dynamo in incapable of producing more current than the LED can handle.