So, you're comparing the performance of a game yet to be released with something that was released over nine months ago... OK...
I'm not saying you're completely nuts here. There are a -few- places in the single-player demo where my framerate tanks too, but you're playing with a 128MB card where 256MB is now the standard for most video cards. I'm also using things like onboard sound (and not a particularly good onboard sound comparatively) so I don't push the settings up on Audio.
If it's not a limitation of your hardware, are you running with the latest drivers for your video card? I don't know if this is the case for F.E.A.R. but I recall that Monolith had a partnership with ATI for the promotion of The Matrix Online during E3 two years ago. Maybe the game is better optimized for ATI cards and only slightly less so for nVidia? I haven't read anything about an ATI partnership for FEAR, though, so perhaps this isn't the case.
Are there certain areas of the demo where the game gets chunky or is it pretty much the whole thing?
With new games like this, the mantra always seems to be "Computer game software sales help push hardware."
My old Gainward GeForce4 Ti4200 worked great on games like Asheron's Call 2 (which, believe you me, was a serious resource hog), but could barely run MxO even with the settings turned way down.
In other words devs use the latest cards to make their games look better than the preceding stuff, but sometimes that means the consumer has to go out and get a new video card to see good performance from the game.
Carmack was the main player in this game for the longest time, but other developers caught on.