FarCry will run happily on a 1GHZ/256MB/GeForceFx 5200 system (at medium everything 1024x768)
Doom3 will run alright on a 1.5GHZ/256MB/GeForceFX 5200 system (High - 800x600), but for Doom3 i would STRONGLY suggest 512MB Ram.
Loading times half (literally); if you have 2GB, you'll notice Doom3 loads are as quick as those from Half-Life 1 (which is pretty cool).
theif3 you want an NVIDIA card. this isn't me being a fan, it uses a HIGH ammount of volumetric shadows and nvidia shadows. you know there is a speed difference when a GeForceFX 5600 can outperform the 9600XT heh
not tried blood rayne 2 yet, it's on my Friday download list though
... can't say no to a game who's advert has her leather clad ass as one of the cool new features