Umm you probably can run some compatible debian applications from desktop but it is not optimised for the N900 and it will be painfully slow to use. You can download the compatible package compile with arm support and it will work. It may be not optified but you can use it like the desktop. N900 can never reach your desktop level in term of power and RAM. So bare in mind that it's running an ARM processor not Intel or AMD. Recompile the code may be needed to make the package install to the right directory.

Yes you can run GiMp and openoffice, firefox and many other one as well. But not everything just some.