F.E.A.R is the best and scariest. But it's not out yet
While you're waiting for that, check out Alien vs Predator 2. The marine campaign is indrediby tense. I've heard that Clive Barkers Undying is a good scare too, but I've not played much of it so I couldn't say.
Thief 3. It's not entirely horror, but the series does have some of that aspect. Some people are turned off by the gameplay because they don't understand it. Garrett is a rogue--a thief, not a warrior who slashes through like a first person shooter. You have to use shadow and light to your advantage.
I cant run fear with only a Radeon 7000 series graphics card,demo didnt even work for me.I got avp which doesnt really scare me unless you play single player with human and the alien just pop out of nowhere.I did used to like thief when pt 2 came out.I thougth it was only for xbox.Ill go look up every game you guys post thanks for the help.