This fix by Slothie ensures that Clear Sky is using all 4 cores of a quad core CPU.
Insert File into the Clear Sky Directory and execute. It should launch Stalker - Clear Sky then wait 30 seconds and set the affinity on the xrEngine.exe process to use four cores instead of just one.
This helped make the game run smoother for me, I even got to set my graphics a little higher. I only tested this on my PC (Q6600/Vista 64bit)