I have been quite happily using a 32GB SDHC card in my n800 for a while. It has over 6000 mp3s and 1900 ebooks (epub format, I use FBReader for them). The tracker daemon takes quite a while to index it all in the background on startup, but it's otherwise fine. The supplied media player can get a bit unresponsive with so many tracks (my old Rio Karma was fine) so there's another music player modelled after winamp (I forget its name at the moment) which seems to be a bit better.