Amnesia: The Dark Descent is easily the scariest game I've played in recent memory and sh
its all over what
Dead Space tries to do. Horror is a hard thing to do and it's hard to quantify what makes something scary and what makes something that tries to be scary, not scary. The problem I have with
Dead Space is that it's less about horror and more about startles. There is some decent atmosphere in the beginning, but it never really builds on it, and that's largely because of the combat. Combat and survival-horror go hand-in-hand, but with
Dead Space it's all about dismemberment, and in order to properly defeat an enemy (which you're suppose to do), and with that kind of specfic combat style, you're forced to constantly have a good idea of what the enemy looks like, and that's a problem because your imagination is no longer doing the work.
Conversely in
Amnesia: The Dark Descent, you're always unarmed so there's no combat, you have a very finite light source, and you can never see the horrorable shambling monsters that persue you, and if you do your sanity drops and controlling your character becomes troublesome which makes you even more vulnerable. Hell, you don't even get the idea that you're being persued by anything until a good 30-40 minutes into the game, but there's always this chill up your spine. That's good pacing because your mind is already conjuring its own idea of what's scary, with the added bonus of never really getting to see the monsters, those conjurations are always with you, and nobody knows how to scare you better than yourself.
If you have a PC that can handle
Amnesia and like horror you should definitely get it, just be warned if you have Intel video cards you won't be able to play it, but if you have NVIDA or ATI you should be fine.
It's not going to be a horror game at all, it's going to be straight action.
The game has to sell 5 million copies just to justify its development costs so you can bet that they're going to try and tap into the full 3rd person shooter action market by junking as much of the "horror" elements as they can justify.