Assuming 1920x1080 0x AA 16x AF
1. Mafia 2 (specially) 6850 or above, I guess one or two of the settings will need to be off
2. Crysis 1 6950 CF or above
Crysis 2 GTX 570 or Radeon 6950 2gb or above
3. Just cause 2 Not sure but I guess 6850/6870 should be enough
4. Gta 4 6950 2GB or above
5. Nfs run and few upcoming games. With all settings & eye candy. 6870/6950 depends, since the game hasn't released. But safe is 6950 2gb.
I am assuming you need an avg of 50-65 FPS in these games with the minimum around 40 or so.
If you want with 4x AA (not MSAA)
MAFIA 2 , 6870/6950, not sure about 1-2 settings if there are any which are very taxing
Crysis 1 , 6950 CF
Crysis 2 , 6970/6950 OC
GTA 4, 6950/6970 2gb
NFS RUN, 6950 CF
--- Updated Post - Automerged ---
Extend your budget a bit and go for 6950 2gb for 16k. Also, you will need a high quality 650-700 watts for such a rig, to be safe and to have headroom. Something from Corsair or Seasonic is required with 650+ wattage.