Well. I've written to the store I bought the card at, and in case my old card works fine I'll send the 760 back.
Regarding Windows and the HDD issues... I got a basic idea of what I want to do: Buy two new 1TB discs. Put them in and let D:, which contains all games and important documents, get RAID or whatever it was called and mirror all the content over there, or at least the most of it. After that I will install Windows on the empty 1TB disc, and try to set up everything. If anything goes wrong, I could always boot my old Windows for the meantime.