Hi all who helped a lot:
I examined your suggestions and decided that Pivot Root Install is just what I need. Unbelivable possibility were it not true.

Preliminarily I tried on my i386 (drawback: live knoppix501 refuses networking; on my amd64 it networks, though I can not use the machine because APC broken and HD raid too expensive to be used without protection). So I have to proceed with i386.

On sda (unformatted Samsung 80 GB), following
#knoppix-installer
I used qtparted., though I was in doubt how to go on because of my limited knowledge (I only know how to use debian installer, raid or no raid). Well, I devised to partition as follows:
/ (root) (ext3 10GB)
/swap (linux-swap 3 GB)
/usr (ext3 10GB)
/var (ext3 3GB)
/tmp (ext3 3GB)
/home (ext3 30GB)

Now the doubts:
I set up /, /saw, and /usr as primary partitions, filesystems as above. I thought I have exhausted possibilities for primary partitions and went to extended partitions. I could just set one (with no specification of filesystem). Then, the remaining free disk refused to be partitioned further.

The question is how to modify my obviously wrong procedure and if the stated partitions are advisable, or better just a couple of partitions. I am used at that partitioning scheme on debian, though I can change if necessary.

Once the system is up, do you expect that networking difficulties on my i386 may be overcome for apt-get?

Thanks for your exquisite kindness

francesco