For playing games smoothly at very high or ultra settings, you really need a good system to handle them. And mainly it depends on which screen resolution are you going to play all the modern games. For example, if you have a 720p resolution (1280*768 or 1366*768), you will at least need 8 GB of DDR3 RAM. And for playing at 1080p resolutions, you will definitely need 16 GB RAM for playing all the graphic thirsty games at ultra settings. Besides, you will also need a good GPU like the one from ATI R9 series or Nvidia 700 GTX series. And to maintain all this high end system, you will need a good PSU (power supplying unit) which should be rated at at least 500 - 600 watts.
PS: I recommend you to buy only Corsair DDR3 RAM and Corsair 500 watts PSU.