Which memory sockets on the motherboard did you install the memory modules into?
Since you're only using one dual channel memory kit the two memory modules should be installed in DDR3_A1 and DDR3_B1 or in DDR3_A2 and DDR3_B2.
There should be one empty memory socket between your two memory modules to be able to operate in dual channel mode.
If the two memory modules are installed right next to each other they will be operating in single channel mode only.
Thanx, I did that and my ram is Dual now.
I reset the Bios too (I did it before and didn't work), and now my cpu core speed is always between 1800-3600 MHz, but when I start any game, it drops again to 800 MHz. One friend has tested and he has always 3600-3900 MHz even playing. He has the same build as me, instead of the GPU (he has an Nvidia GTX 660) and the same model of ram, but just 1 of 8 GB