It will bug you constantly to activate it, until you do. Both of my copies of Windows have been upgrades from OEM versions of Windows 7.
I know how you feel confused as you often read about people having Windows on an SSD and they're files on an HDD, that is, however, as said, for fresh installs.
I don't know what is taking up most of you're space, tbh. I've got a 500gb and I'm only using 150 gig including the OS. And I've got music, games and videos.
Once you get you're HDD down to at least 240gb, you can use a disk image tool to image it to the SSD. I used Acronis disk image for mine.