4gb internal
Music, movies, ebooks, and maemo-mapper files. I don't worry much about the wasted space from using a larger cluster size on maps - I have less than 1GB of maps right now.

I keep a second smaller card (1gb, right now) in the external slot, to move data on and off easily via a card reader. I usually use SCP from the desktop to the n800 to populate movies and music - just habit, no really good reason!