make sure Maemo-extras is listed and active (it should be, but you never know). this is where a lot of the fun stuff is saved (i'm not going to go into detail, you can google "N900 repositories" if you need help there).

You should be able to simply run a search in the application manager for "Dosbox" so long as you have the Maemo-extras catalog active, it should show up.
(I recently upgraded to PR 1.2, and Dosbox doesn't show up for me, so if you already updated you may have to wait until Dosbox is compatible again)

You can use the same steps for downloading other emulators (DrNokSNES, PicoDrive - for Sega games, VGBA for gameboy advance, and others)
You'll need to get your ROMs from somewhere else, just google for them and you'll find what you're looking for.

If you like old school adventure games, download ScummVM. I can't speak for performance on the N900, but its very good at playing older adventure games created by LucasArts (Day of the Tentacle, Full Throttle, etc.) and similar adventure games. You will need to have access to the original game CD (or its files) for you to play any games, however.