Total Available memory is much lesser than Physical Memory

Hi, I have E6510 Dell Latitude. I have installed 64-bit Windows 7 and have 8gb ram built into it. I see that when I open up my task manager, it shows 

Physical Memory Total: 8181

Cached: 5733

Available: 5774

I have 512 mb Nvidia Graphics Card installed. Even if I exclude the graphics memory I should atleast have 7.5 gb available.

There is no "Memory-Remap" feature built into its BIOS. Can anyone help me out with this issue of increasing the Available memory