1: I've seen only two brands of 16GB SDHC mentioned here by users, one is the A-Data and the other is the Patriot (the latter is more expensive). Both are reported to work, can't say I remember anything about speed for the A-Data but the Patriot is reported to be fast. My guess is that the A-Data will be fast enough, as most new/high-capacity cards are pretty much hitting the N800 speed limit anyway.
EDIT: I forgot the Transcend 16GB card, mentioned below by another poster - it's been mentioned here on ITT before. So, three brands (so far)

2: Presumably.

3: Don't know - someone actually using KDE will have to answer that one.