Stalker Shadow of Chernobyl + Complete + SC Realistic Weapon add-on = incredible.
I'm one of those guys that liked the original Stalker just fine. Adding Complete made it one of my all-time favorites. The shooting is better, Barkeep can repair your weapons and armor, and you can sell loot to nearly all stalkers you run into, and most of them have at least a few thousand bucks on them. This way you don't have to keep hiking back to the main traders to sell off all your loot. I seem to be finding more special weapons, and it's easier to buy special ammo. Not to mention massive changes to the graphics, weather, ambient sounds and all that jazz.
Playing this at night with my headphones on was a freaky experience.
Oh yeah, playing on Master difficulty is recommend for better balance with this combo of mods.
It all worked just fine on Steam too. Ran quite well on my old PC, can't wait to see Stalker on my freshly ordered Digital Storm rig. :)