I've managed to put together a brief F.E.A.R Demo Tweak Guide, the main aim of which is to help you get the game up and running and performing well on your machine. That way not only can you enjoy the demo now, you will also be able to better judge how the full retail version of the game will run on your machine when tweaked, and hence whether you really need to upgrade or not for it.
TweakGuides
TweakGuides