iTunes should keep the albums separate by default because, although the album name is common, the artist name (year, composer, genre, etc) is probably different so it shouldn't be grouping them together.
To make sure that iTunes knows how to separate the "Greatest Hits" albums from one another:
Select all the "Greatest Hits" tracks that belong to one of the artists only, Get Info (cmd + i) and in the Artist name type whatever the artist is and check the "compilation" check box in the get info panel too.
That way it shouldn't mix all the Greatest Hits albums, unless you're browsing for album name. It should now recognize that the albums have the same name but are from different artists.
And to get Album Art you can always check out CoverScout, which you have to buy, but there are a lot of free apps out there that do the same, just google it
