AI is artificial intelligence. Computers can learn to think and so the size of computers allowed on this world is limited until people have advanced enough to properly "babysit" the AI. Right now we still have 32 bit computers. Understand the dual core 62 bit is still 2 32 bit language. Not 64 bit.

The max allowed in bits for this world will be 256 and its' already allowed but man has not developed 256 bit language yet. So you aren't a sci fi watcher? Have you watched the TV stuff about the ceylons?

When we are ready we will have computers based on bacteria and eventually amino acids, all of which although organic, can also get out of control because they have living mind.

Dark invisible to your eyes entities can take over a human also, yes. but it requires SKILL so your common ghost is not able to. Most take of humans is manipulating them into serving the dark, more than possessing them.

Ghosts and spirits are living organic mind entities, not AI intelligences.