As with many questions the answer to "What size memory card will I need?" is: "It depends".
It depends on how much you value images in an encyclopedia,
on how future proof - or rather future resistant ;) - you want to be, and of course on your budget.
|Handheld memory needed|
|Small images 2005 PPC/Palm||2 Gb||-|
|Larger images 2005 PPC||4 Gb||-|
|Text-only 2007||4 Gb||2 Gb|
|Images 2007||16 Gb ? *||4 Gb|
|1 Gb is a gigabyte is a billion bytes|
*Rough estimate, not produced yet, expected end 2007.
Here are some considerations:
Text-only edition: the December 2005 text-only edition of the English Wikipedia was the last one to fit on 2 Gb memory card. You'll need a 4 Gb memory card for the latest release. You might need twice that much memory or more in a year from now.
Edition with images: if you want images, you'll want the largest memory card you can afford (and which is supported by your handheld OS),
meaning at least a 2 Gb card for the English Wikipedia of 2005 and a 4 Gb for the German Wikipedia now, preferably more for larger and more images, and certainly more in a year from now.
Memory card prices fall steadily: in 2005 I bought a Sandisk 4 GB Ultra II compact flash card for less than the price I paid for a 1 Gb Sandisk CF card two years earlier. Now a 16 Gb card would cost less.
By then compact flash was easily the most economical option, but nowadays compact flash and secure digital memory cards are nearly equally priced. Quite a few pocket PC's (not sure about Palm) support both card formats, which is one of the reasons I'm quite satisfied with my HP handheld.
Of course the English Wikipedia form 2005 is already a massive reference work,
larger than any paper encyclopedia, and almost as authoritative as the best among those, according to science magazine
Most areas of general interest and advanced topics are already covered in depth,
so if you choose to buy a card which accommodates that earlier Wikipedia edition without much capacity for future updates,
you will still value the content in years to come.