well, unless there really are specific packages in the debian repository which you absolutely must have, i would again recommend against trying easy debian, but if you're feeling adventurous, here are the installation instructions:
http://wiki.maemo.org/Easy_Debian

i don't know if aptitude or synaptic are available, but if there are "armel" packages for them (that's the processor architecture of the N900), chances are they are gonna work.
but i haven't installed easy debian yet, so i'm only guessing.

be prepared that you're leaving "end-user territory", which means lots of commandline and reading documentation and how-tos.
you can of course come back here and ask if you have a specific problem, but don't expect experienced users to explain every little step in detail. you'll have to invest time and sweat yourself