What is the most powerful computer that can feasibly run off of two (or four, at most eight) AA batteries?

I've considered getting a Game Boy Advance or Wonderswan so I can play those games in their intended environment: but I'd prefer something that's more general purpose, complete with a keyboard.

As for battery life, I'd expect the thing to last at least 4 hours.

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Psion Series 5 - Wikipedia

@WiteWulf @moses_izumi Yep. 5mx is the most powerful.

There's also the 3mx. It's an older generation and less powerful - 16 bit Intel rather than 32 bit ARM4 - but still good at 27 MHz, with a lightweight OS. It's still a great machine. I use mine all the time.

You'll easily get 4 hours out of two AAs. If you use Lithiums, you'll probably get closer to 10.