I’ve received some questions and would like to post the answers right here in case others are wondering the same things about the N900:

• Yes, the memory on the device is partitioned so that you have around 1.5GB or so for applications, and the rest for data (images, videos, music, maps, software you use with DOSBox, etc.)
• To install Dosbox you have to add one or all of the extras repositories in your device App manager: extras, extras-devel, extras-testing. More instructions about that are available at: https://wiki.maemo.org/Extras