Battery-powering up a Beagleboard XM
Because of regulator dropout, it will be hard to find an ideal regulator that can deliver >1A from four NiMH cells.   Low dropout regulators typically have modest power, high power regulators typically have larger dropout.

I recommend you switch to 6 cells (7.2v); however I just don't use NiMH any more, lithium cells are so much better.   Search for "protected 14500" on dealextreme.com, these AA-sized 3.4v laptop cells are awesome.  The larger 18650 size is good too.   

For 5v regulation, try the "Battery Eliminator Circuit" from a  model helicopter (search for BEC or battery eliminator on dealextreme.com or a hobby site such as hobbyking.com).   These regulators can handle way more current than you will use. eg http://www.dealextreme.com/p/8s-5a-switch-mode-ultimate-bec-ubec-45214

Don't forget you can use your multimeter to measure how much current your beagleboard
is actually using, which will let you know both how beefy a regulator you will need, and
how long a battery life you can expect (factoring in the efficiency of your regulator).