Have you played STALKER yet? When it comes to atmosphere the game is unbeaten, and the combat and AI is very good as well. Also, you may want to try Hitman: Blood Money. The best of its series, mostly because they have the stealth AI down perfect and have been improving greatly on level design. No ultra-linear forced stealth levels like in Codename 47, no short filler levels like Jacuzzi Jacob and Hidden Valley like in Silent Assassin, and no unoriginal remade levels like there were a multitude of in Contracts.