Here my second system's specifications:
Same CPU (Winchester)
Same Memory
ASUS A8V Deluxe
ATi Radeon 9800 Pro 128 MB 256-Bit
I could play F.E.A.R. on that system at 1024 x 768, most options at high or highest, with Shadows turned Off, as well as one or two Lightning options turned Off (don't remember). Everything else was high, or to their maximum settings.
At 1024 x 768, even though some options were on high, the game could be played without problems. Between 25 to 40 frames per second, if I remember well. It was playable, very playable.
But at 800 x 600, I could turn most options On, including the Shadows (with still one or two Lightning options at medium or low I think), and I could play it at least at 30 frames per second, and went to 50+ in certain areas, and that includes the firefights.
I'd give a 6.5 / 10 in terms of performance and playability with still a good overall visual quality and use of the highest variant of the physics and particles in terms of in-game options.
So, on a 9800 Pro it's quite decent enough I'd say.
I still prefer to play it on my host (main) system though, of course.