4870 will max out all those games without any problems. 1GB would be good for bigger monitors, 512mb is good for your monitor size.
I have the 1GB version and I play FarCry2 (far more gfx intensive than the ones you listed, except Crysis) at max settings and AA at x4, and still get around 50-60fps average. This is with a 1920x1080p resolution.
You can basically max out any game, and play Crysis at Very High with lowered AA at your resolution of 1280x1024.