When you buy batteries aim for something high in the mAH like 4500 or 5000. Oh yah and make sure it fits in the case. I couldn't fit mine.
On the lines of not frying the thing. Make sure your getting 3.3V from your regulator. That is the only thing that has fried my N64s.
You need to get Ben's book to learn how to make a case.
Here's the diagram for the DC/DC regulators for the 3.3V linehttp://benheck.com/phpBB/viewtopic.php?t=15781&start=0 this should also provide some useful info. Just scroll down and have a look. But first check this page out http://benheck.com/phpBB/viewtopic.php?t=7437
