Perhaps it's been noted before, if so; excuse me.
The Kindle has 4 GB storage= 3 GB for books/doc/audio + about 1 GB for its 'own' functions, like searching, indexing etc.
Try not to exceed the 3 GB that is left, as the Kindle will go unstable:

- doesn't search anymore: says ' needs a minimum of 25 KB for searching' ,
- keeps asking ' Turn Wireless On'
- starts to index all of your items all over again
- battery empty in one day, etc.
That's what happened when I had 3,24 GB on the Kindle. It was really the limit. It went unstable and almost unmanageable. Rebooting didn't help either, of course. I deleted some audio (PC) and a lot of books (through Calibre) and it's allright now.
So; check your GB's on the Kindle once in a while and keep to <=3GB.