Well I didn't do the first mission ghost, but as soon as I learnt of the chaos level I didnt kill or be seen by anyone. I guess the only exception was the Boyle manor but I made sure not to be spotted at any point trespassing. I can't remember if that still gives you the ghost rating. Sleeping darts and choking people still gives you ghost rating too (although, I found it more fun to never use sleeping darts).
I also spent about an hour killing everyone in obnoxious ways before reloading a save point to carry on and did a lot of general exploring so I feel like I got pretty much all I will out of the game.
Edit: sry I just read nymethus' post. I played on hard straight away. I don't think it makes that much of a difference if you stealth through it. Oh and I was really obsessed with the game's lore so I spent awhile reading every book I came across until I realised most were duplicates.
