I know that the 4200 Go should play the game resonably well because Doom 3 plays great.
You can't compare the performance of games like Doom and FarCry/HL2 to each other. Just because doom runs good doesn't mean FarCry or Half Life will. Why? Because Doom is OpenGL and FarCry is Direct3d (HL2 is also D3D). OpenGL games always run much faster than Direct3D games on nVidia hardware. Just check my sig for example. My doom3 benchmark is with 8x AF. My FarCry benchmark is with no AF and is still lower FPS on average. You're gonna need more power for D3D games, thats just how it is.