it simple with 4g of ram you lose 750k of it to the intel mb leaving your system with 3.2g of ram to start loading windows.
windows is going to take a chuck of that ram for the os and your anti virus program and video control panel and the audio control panel at start up. then windows is going to set some of it up as a swap file/min ram drive. what left is the ram left for programs and games. with 4g of extra ram windows and the game can load a lot of the game into a ram drive so that there no cpu/gpu bottle neck.