Would it be possible for a ghost to gain a body using the same sorts of rituals that Voldemort used in Book Four? I know that Dumbledore told Harry that it was impossible to bring the dead back to life when they were talking about the "echoes" that came out of Voldemort's wand, but is a ghost really that different from the spirit that Voldemort was?

 

Probably could, but Voldemort was already of flesh when he was ressurected.
I don't know if Spirit energy could be made into physical matter.

Though I suppose that the ghost could in theory possess a persons body.

I have read an article (I will find it) that states that in theory, two souls cannot inhabit the same body.

So the corpse in which the ghost would be ressurected would have to be that of a newly deceased person.
But then there would be another ghost that needed a body.(I think?)

 

Well, Voldemort possesses Quirrell, all those snakes, and then Harry, so I guess two souls can be in the same body.

But would a ghost be able get some kind of body like Voldemort had at the beginning of The Goblet of Fire using unicorn blood, snake venom, and whatever else Voldemort did and then do the ritual that Voldemort did at the end of The Goblet of Fire to get a real body?

 

Not sure, but the article I will find for you (eventually) was apparently based on scientific research on the recently deceased and theories of the soul relating to quantum physics, not the works of JK Rowling.
In terms of Harry Potter Fiction then surely anything can be done can't it?

 

Well, while they can inhabit the same body, they tend to die fast. It says in Canon they all died fast.

 

Voldemort is more than a ghost though isn't he, he's still tied to earth by his horcruxes whereas a ghost is actually fully dead and just hasn't passed on.