A123 packs are the way to go, they charge fast put out a high voltage that stays steady, and you don't need a reg
I agree that the A123 is the best way to go, but UNREGULATED?
If you go 2sXp, you get 7.2v when the baterry is toped off, agread that some servos are ok with that,
to me that best part of using regulated output voltage is that:
1. I can be sure I'm not stresing my eletronics that much
2. I can use "4.8 only" servos safely(just not on gyro servos, in gyro aplication)
3. The servos speed is constant if it's the first flight of the day, or the last (Not that I would be able to notice the diference