thanks all of you.
I think I can handle it like this:
0. I had already updated the firmware and OS in the flash
1. I will Clone Os
2. install app to be able to gain root
3. download the repartition tool to make one or two more partitions.
any suggestions on the size of the partition used for easy debian?
4. install easy debian following the instructions.

let me imagine, please tell me if i am right:
I use application manager to install easy-deb-chroot and then run Debian Image Installer in the Extras menu to install easy debian. if everything goes well, the easy debian and applications should be installed in proper directories which are created by the installer automatically.
(are there many directories created or everything is put in the directory called /debian?)
5. after that, i will follow Qole's instruction to
First, from inside Debian chroot, issue an "apt-get clean" command. Then, from a root maemo prompt (or add "sudo" in front of the mount command):
Code:
mkdir -p /media/mmc1/debian-apt-archives/partial
mount -o bind /media/mmc1/debian-apt-archives /debian/var/cache/apt/archives

please kindly correct me or confirm

another question: the N800 uses WPA and my home wifi is WPE. Can i connect N800 into the network? I forgot keys so I had not tried yet.