Thanks Rob1n.

A few more questions:

1.
Is there any remote possibility that there are packages installed on my n900 by app manager or nokia that can be safely removed without jeopardizing system? If so how can I find them?

2.
Is there a list of groups of apps that will fulfill most of task needed by certain activities that will minimize consumption of rootfs?
Let me explain what I'm leading to:
I want to install latest port of Qt and try it out with some applications that are available in Qt flavor but in GTK+ ports yet (high pathetically). If I install current build it will bring my rootfs to the max. As I already know - this not good. So to resolve this and get what I want I need to find out what I can remove from my system in order to get Qt goodies and still be able to perform tasks I need to do on daily basis.

So it would be good to have some wiki page that will describe to curious noobs like me (who would like to try more before getting attached to something):

1) rootfs limitations (warn us on the possibility to basically render your device unusable by installing too many apps)

2) a brief explanations on GTK+ vs Qt (and consequences to use either one on ones device)

3) list GTK+ based apps (including the one native to maemo 5 image that is factory installed) AND their Qt based counterparts

4) specify instructions of needs to be done to replace GTK+ based apps with Qt based (and vice versa)

This will be helpful even in case that there were no issues with rootfs space limitations.