One mod I would highly recommend is STALKER Complete 2009 link here: http://artistpavel.blogspot.com/
(for Shadow of Chernobyl only)
Read the list of all the modified and merged mods, it is quite impressive all the stuff that is included.
Once installed if you go to: THQ > STALKER-Shadow of Chernobyl > gamedata > shaders > r2 > SkyGRAPHICS_options (open with notepad)
You can modify/turn on/turn off the improved motion blur, depth of field, parallax oclussion, SSAO and sun shafts (god rays) to your liking.
I wouldn't mess with sun shafts they are off by default in the mod and can cause strange graphical glitches if turned on.
But are pretty to look at for a bit, just to see what it does even if you don't leave them on for good.