If your version of WordPress lacks an export feature (Manage > Export), there is a plugin which let's those running older versions create a wxr (WordPress eXtended RSS) file:
If you have access to the database through a tool like phpMyAdmin, it's also a good idea to get a full backup of the database as an sqldump file. One can never be too extreme about backups.
As for your question about WordPress.com, I've no idea what their limit is nor if they offer other alternatives. I'd ask them about that on their forums: