I've mentioned System Shock 2 before; to make a long story short, it's one of the finest computer games ever made.

The random Gnome's random Lair published an article on how to run System Shock 2 on modern PCs, plus links to the various (extremely mature) mods that improve the sound, texture and mesh quality in-game; one of these mods also subtly balances the game, making certain items more powerful while normalizing others.

I've been playing System Shock 2 once a year or so since 2003 (four years after it came out!) and I can tell you that it's an experience not easily topped. Go, play!