Well if you have no budget, a 6950 would be a great choice. But if you're only playing COD and Minecraft, you won't need that much power. And, speaking of power that 400 watter won't power a 6950. So, either upgrade the PSU or you'll need to get a lower power GPU - 6770/5770 perhaps.
Will this GPU run the games listed above, at a playable FPS, NVIDIA GeForce GTS 450 1GB