Would you be able to hook th Pi up to a computers motherboard?
I was wondering because of the small amount of resources on the RPi and wondered if it would be able to tap into the RAM and CPU on the mainboard as well as the HDD/SSD or Optical drive
Simple answer? No. Considering the Pi and traditional desktops nowadays use completely different architecture, there's no way you can do a simple 'plug and play' solution.
Even if you were to consider it, you would most likely need to re-engineer the connections and the boards on both computers to be able to 'hook in' and that just becomes unfeasible because at that point you're better off just using a regular computer to do what you wanted to do.