The first time you enter the wild territory (and then onto yantar, etc) the atmosphere just hits you like a ton of bricks, it's almost palpable and no other game since has come close to it in my opinion (except maybe the metro games, but they still have this artificial feel to them).
The thing with Stalker is that it's not just the graphics, but every other aspect of the game, like the random background noise, the distant growling and screams, the brooding dark ambient sound track, it all just draws you in and makes you truly feel like you are in uncharted territory and could die at any moment.
I can remember first playing it on my x1900xt 512mb, playing it at 1280x1024 res, barely getting 30fps, yet it was and still is probably the best gaming experience of my life. What a game.