The best game that I've ever run was a 5 to 6 month long Ravenloft campaign set in the Pathfinder rules and in Golarion. I ran it last year.
Ustalav is essentially Pathfinder's version of Ravenloft, so setting the game there is a decent idea. In that vein, Rule of Fear is the Pathfinder sourcebook for Ustalav and is valuable if you want to play there.
My game was extremely low combat, and I find it difficult to affect the "horror" if you add too much combat. YMMV.
My players were told upfront that they would gain levels at the rate of one level every two weeks until level 3, at which point it would increase to one level every 3 weeks. No experience points. This allowed me to really tailor the encounters which is fairly essential in horror, in my opinion.
I used a lot of music and it helped a lot.
I heavily encouraged (almost forced) my players to create characters with horrible secrets and then I exploited them to the max.
Lastly, sources... If you head to
Secrets of the Kargatane: The Home of Ravenloft on the Net and check out the netbooks and adventures there, you'll get a lot of good ideas. I used material specifically from Forgotten Children and Children of the Night: Demons. Those netbooks are really light on monsters and really heavy on characters, storylines, and atmosphere.
As for the Carrion Crown AP, I played through the first two books (as a player) and I wouldn't recommend them. They're good adventures, but if you're really looking for horror then you won't find it there without a really, really good DM. Even then, the first book is particularly complicated and reads well but is difficult to run, IMO.