And I don't want to upgrade because I really don't play new games much or do intensive work, but I have a problem because my case doesn't support multiple hard drives and I want TONS of space.
Do I just buy a new case + HDD or is it better to just build a new PC in general? I'm a pretty poor gamer, all in all, because I still use VGA to output to my monitor and stuff like that.

I currently have a 1TB HDD which is very much out of space for what I want to have on my PC (a huge fuckton of anime, manga, video games, art, music, books and a lot of other stuff).
The thing is, I still have space for an SSD, but is that a smarter choice or what should I do?
I've also been looking into those tower thingies, which have like a shitton of drives in them and act as a home server or whatever.
I ideally want at least 8TB of space for now, so what is my best and cheapest option with only storage in mind?