I know how ya feel, Cheif. I've always loved realistic shooters like Rainbow Six 3: Raven Shield, etc but they do get old. Latley i've been playing more of the popular games like Half-Life 2 and Ut2004. I just had to take a break from realism.
One great - REALLY Sci-Fi game you might want to try is F.E.A.R. It's got a really gut-wrenching (Scary) single player storyline. It's like you're acutally playing a horror movie. Check it out:
http://www.whatisfear.com/us/
Another Sci-Fi game that i'v really been looking forward to is "Stargate: Alliance". It's based off of Stargate SG-1 on T.V. It's still in development, but it has been for a while. Hopefully the game will be worth waiting for!
It's official site:
http://stargate-alliance.com/index.php?flash=1
And here's a great fan site (Lots of inside info):
http://www.stargate-gaming.com/site/
I hope you find a good, new game! I highly reccomend FEAR. Cheers!