Haha, another Quake 4 fan. I thought I was alone.
Check out Prey, it runs on the same engine as Doom and Quake. Hmmm try Crysis? It's sci-fi of somesort though you fight humans in the beginning.
I'm checking gamespot right now for Sci-Fi games and one with high scores is Dead Space.
dead space can be a truly creepy game, especially lights out and headphones on.