Your n800 is running an old version of OS2008 (Chinook) that didn't have the SSU feature which is why you're not seeing any updates.

Like GA says you'll have to flash your device to upgrade to Diablo. If you do that then get the latest version 5.2008.43-7.

Flashing will wipe your device so backup anything you wish to save first.
Caveat: Some reports suggest it's better to start with a clean install and not restore backups from an older OS to avoid possible problems.

In any case it should be the last time you need to flash as any further upgrades to the OS won't necessitate a complete flash of the n800.