8GB RAM But Ubantu Shows 4GB

AD76XYZ

Honorable
Jul 17, 2013
11
0
10,510
I have following config.

Processor...
AMD FX-8350

Motherboard....
ASUS M5A78L-M/USB3

RAM....
Corsair Vengeance 4GB X 2
DDR3-1866/PC3-15000
CMZ8GX3M2A1866C9R

Graphic Card....
ASUS 5450

When I set RAM speed 1333MHz Ubantu reports RAM some 7.8GB.
But when I set RAM speed 1863 in BIOS then Ubantu reports RAM some 3.64GB.

Can any one tell me why this.
How to correct this issue.