Doom 3. Fun. Fucking scary. And not just pretty graphics.
Obviously it stresses your graphics card, but it's also very CPU intensive. I AM going to upgrade my CPU, asap, just so I can play this game better. I've been meaning to upgrade for a while anyways, but this will just be a cheap patch job. Patch or no, though, it'll still let me run the game at more than 640x480 on medium detail smoothly. :P
Anyway. If you're a FPS junkie, I can't tell you how much you need this game. Plug in 5.1 surround sound and play in the dark. Or headphones up loud. And in the dark. Either way, you'll be shitting your pants as you run low on ammo and the very walls seem to exude malevolence. A must-have.
*fights the urge to play to go and do some hardware price research*