I recommend using the stock backup program for application settings, contacts, calendar etc, and BackupMenu for the rootfs/optfs. You will need to copy the backups off the device before you proceed.

Once you've done that you can repartition the EMMC without wiping everything. You reboot the n900 with BackupMenu (by leaving the keyboard open) ang go into USB Mass Storage mode, and then plug it into your PC. You need a partitioning software to do the rest. I used GParted on Ubuntu. Here's a link for using a LiveCD.

Edit: the image you are using is a combined MyDocs/Optfs partition. So yes, you would need to get hold of a firmware image this page has all the info you need.