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Dillo would be the smallest but Opera would be the best for features per KB of RAM.

I would personally use Dillo because it just seems more suited to the environment and would fulfill it's purpose (looking up documentation) without crippling the system.

If you wanted to just browse the internet though I think Opera or Chrome would both be good choices. Especially if you used a minimal GUI.

Something like uzbl[1] or conkeror[2] maybe, i.e. a very small wrapper around the rendering engine (can't think of anything beyond Webkit or Gecko that wouldn't completely annoy users).

[1]: http://uzbl.org/

[2]: http://conkeror.org/

PandaBoard...if you can get one. http://www.pandaboard.org

If it's anything like the BeagleBoard, just place an order for a single unit (even though DigiKey likely lists 0 in stock), and you'll get one fairly quickly (probably faster than the 4+ weeks for a SheevaPlug).