You can format memory cards in the N800 - in the File Manager application, open the application menu - press the Menu button while the File Manager application is active, or click in the application title area with the stylus (the area to the left of the system tray with all the icons).

In application menu you should see a "Memory Card" menu with options to format or rename the internal/external memory cards.

If you are using swap, you won't be able to format or rename the internal memory card.