Ways to connect a PCIe/PCIe2 card and other PC parts to a breadboard?
What you are asking is very, very difficult.

You can try to be much more specific and we can try to help you, e.g. in my opinion writing to an hdd might be feasible, but forget to do something useful.

Motherboards exist for a reason, that is connecting all these parts together and providing all the needed interfaces, logical and electrical, between them. That's the adapter you are searching for, it's called motherboard, and it seems quite unlikely to me that you can "cobble togheter" anything similar.

About your graphic card question, I assume you have a basic grasp of how a computer works: a GPU is not even close a general purpose processor, you can write some code for it, some heavy simulators rely on the GPU, but you can't really have anything near working without spending tons of time and money, or without an engineering dream team, and that holds for any definition of working. Unless you just want to turn on the fans, that's easy indeed.