The file free that it reports under Options, Status is the device memory (where messages, applications, themes, etc are stored. Adding a microSD card doesn't change how much device memory you have, but it does give you a better space to store pictures, ringtones and other media files (such as mp3's).
You should use desktop manager to remove any languages you don't use as well as any themes you don't like.
Also, if you go to your inbox, select options, general options - scroll to the bottom and change your message retention period (default is forever I believe).
As your device memory is full, you may have filled it up with pictures, media, etc - copy those over to your PC with media manager, then you can copy them back to your media card.