While it would take a magician to repair the game's myriad AI glitches, STALKER's graphics are more easily improved. And boy, can they be improved.
This is the "STALKER Complete 2009" mod (released last month), which does an admirable job of making the game look like it was, well, made in 2009. New textures, new lighting and new weather effects help get the game looking ship-shape, and while this won't improve any of the game's more fundamental flaws, you can at least enjoy the sunset while wondering how your waypoints disappeared.
Ultro-Mod: Stalker Complete 2009 [Rock, Paper, Shotgun]