How much on-board storage do you think would be enough? I'm guessing Amazon has a pretty good idea of how much storage their best customers need.
On the other hand if I need a certain photography setup for achieving a certain look, I can use Voyage for that - imagine instead of a PDF book with lighting setups and examples of results you had a piece of paper with a drawing - seems pretty comparable quality-wise.
1. crop out page numbers: http://briss.sourceforge.net/
2. remove columns and reflow with: http://willus.com/k2pdfopt/
The later one works like magic, it totally made my ebook reader so much useful. (A Tolino Vision, a very fine german device not locked to the Amazon store: http://mytolino.com/)
(edit: fixed links)