We read the question, Bigmac was just expanding on what I said, and was probably offering out more info to me, and not just to you.
It's my understanding that soft shadows as a whole is a graphics card feature over a CPU. I myself just upgraded from a AMD64 3800+ Newcastle chip to a AMD64 4800+ toledo chip and saw very little in perfomance gain under Fear and Fear XP. I see a bigger jump in performance when I turn off Soft Shadows and Volumetric Lighting.
That implies to me, that it's more on the video card than the CPU. I thought Toms site had an article on this and some benchmark numbers with shadows on and off.