8gb installed but only 4gb usable

[XxAdyxX98]

I recently built my first gaming pc , specs :
Intel Xeon X3440 @3.8ghz O.C.
Hyper 212X
Asus P7P55D motherboard
2x4gb DDR3 one is Kingston and one is Sk|Hynix
Sapphire HD 7790 1gb
3 random harddrives
350w Thermaltake TR2-S 80+

So the problem is , as i said i have 2x4 gb stick in slot A2 and B2 , in the bios it just says 4gb but in windows it says 8gb (4gb usable) so i guess it's all getting recognised but only one is used . Does anyone know what could it be ?

 

[Dunlop0078]

Are you using a 64bit version of windows?

 

[XxAdyxX98]

Are you using a 64bit version of windows?

Yes i'm pretty sure it's Windows 7 Ultimate x64

 

[Dunlop0078]

Right click my computer and select properties, that screen will tell you what OS you are using.

 

[XxAdyxX98]

Right click my computer and select properties, that screen will tell you what OS you are using.

Yep it says 64-bit operating system , and again it also says 8gb (4gb usable)

 

[Calvin7]

Memory is guaranteed in the form sold. Other combinations you decide to make have no guarantee to be compatible together.

 

[XxAdyxX98]

Does someone know something i can do to make them work together , they're both the same capacity and speed , i really don't wanna spend more money on this pc