FEAR, FEAR: Extraction Point, FEAR: Combat (multiplayer one, comes with EP if memory serves), or FEAR: Perseus Mandate?
FEAR is 100% worth the cash. Extraction Point is an expansion that requires the first FEAR to be installed, that ones not worth it as much (5 intervals only as opposed to 10).
FEAR: Perseus Mandate is a standalone, and from where I'm at in it I'm loving it. This one's likely the most pricy though - it just came out the other day. It also requires 7.23GB of hd space to install. If I'm not mistaken, thats more than FEAR, FEAR Extraction Point, and FEAR Combat occupy on the disc TOTAL
My recommendation: If you can find FEAR: Gold, grab that - that includes FEAR, and FEAR: Extraction Point. Around here it's also cheaper to get them bundled like that than to buy them separately
The other recommendation is to try watching some gameplay vids or get the demo; FEAR: Combat (the multiplayer version) is free to play if memory serves, but you will NOT get the FEAR experience from it - the whole point to it isn't to make it a deathmatch. The whole point is to figure out what Project Origin is (or in the latest one, what the heck Perseus is).
I'd definitely spend the $$$ on it. Only thing to keep in mind: FEAR: PM can be a bit cranky when installing - the guys at Sierra screwed up the installer big time, so it likes to install to Drive C:, then copy to whatever drive you actually wanted it installed on. Theres a registry hack available on their own tech support forums to fix that, but once installed it runs like a dream
Hope that helps!