Watch 'Event Horizon' if you haven't already; it'll both scare the shite out of you and give you plenty of inspiration for such an RP.
Think oppressive, enclosed industrial environments for this sort of game. Orbital facilities that smelt the raw materials the Planet Crackers harvest, a colony ship on-route to a new planet. Lots of gun-metal and grit; that's a real staple of series. Pacing will be important from the get-go as well; don't just have Necromorphs appear and then start plugging away at them, build the tension, the suspense, the feeling that there's something really fucking wrong with the environment you're in. The first game did this pretty well (remember stepping aboard the ship for the first time to find it deserted? The blood smeared across the walls? The emptiness?), and if you can pull this off you'll have yourself a good game. Horror RPs are awesome, after all.
If you've got any issues/need someone to bounce ideas off of, throw me a PM; I'm always happy to help.