Thank you for your help, i've kinda understood both the ways. Now, before asking something more, i've two news:
1) Unluckly i've discovered my current case doesn't have enough HD bays to fit two hard drive at the same time (I wanted to have a main SSD and a secondary HDD with all the stuff except for applications and OS) so my project is half of a failure
2) Luckly today is cyber monday and i've found a 256 GB SSD really cheap and a good brand.
At this point, my problem is: i have around 240 GB occupied on my actual 1TB HDD, probably i should be able to free something like 40-50 GB if i put all the unnecessary things in the external hard drive. Would be worth to switch from a 1 TB HDD to 256 GB SSD as ONLY internal hard drive in the pc? However, in about an year, maybe 18 months, i'm planning to substitute also the case, the PSU and other stuff and i will make sure to buy a case that has at least 2 bays.
Except for this question, i have another 2 things to ask:
1) @hamluis basically if i got it right, i have to create a recovery unit with a 16(or 32 GB) USB. Then i will transfer all my datas (documents and other files) in an external drive. Finally i will substitute the internal hard drive with the new one, then i will boot from the USB and clean install windows from the recovery unit. This works also with OEM versions of windows since OEM keys are binded to the motherboard. This way i will have a clean installation of windows, all my files on the external hard drive, but will have to reinstall applications obviously. Is all of this right?
2) @ranchhand_ while i am kinda comfortable with the recovery unit procedure (i think i understood it) i am not sure of understanding how will i be able to restore my old HDD into my new SSD. I mean:
Firstly i'm imaging my current HDD to an external HDD. So now, i will basically have a backup of my current HDD. At this point i should install the SSD, which has no OS in it. At this point i have a PC that will boot correctly but with no OS in it, what do i have to do to backup from the external HDD? Can i boot from it? Sorry if the questions sound stupid or newbie, i always do backups but never had (luckly) the occasion to restore from one of those with a compromised (or totally absent) OS.