If you have a reasonable graphics card and CPU, I'd recommend STALKER Shadow of Chernobyl.
It's sort of an RPG, but it's more of an FPS, it has a lot of bugs and a few crashing issue, but the game play and a strong modification community make up for these facts.
Basically, if you've ever wanted to play an FPSRPG set in post-Chernobyl Russia, this is the game for you, and even if you haven't it's worth getting just because of what it is