Will adding more RAM to my computer be good or just disconbobulate?

[Cludo]

Hello all, I'm not too bright when it comes to Computer Hardware so I though I'd ask you lot.

Basically my computer is using 4GB of RAM which is pretty low these days, Would adding another 4GB of RAM be possible on this machine without replacing anything else?

Windows 7 Home Premium 64-bit
Intel(R) Core i7-2600k CPU @ 3.40GHz, 8MB Cacge, LGA 1155 Socket
Strontium 4GB Single DDR3 1333MHz RAM
Motherboard Asus P8H61-MLE-USB3
Graphics Radeon HD 6800 Series, 2794MB Memory

So if I were to get another 4GB Strontium DDR3 RAM stick of the exact same model would my computer be able to handle this? Fairly sure this Motherboard only has 2 RAM slots too, and I heared using all of them up will decrease memory? What would you suggest? Maybe getting a single 6/8GB RAM stick?

---------- Post added 2012-11-26 at 12:56 AM ----------

Apparently I have a Dual Channel CPU therefor its better and more efficient to purchase another 4GB 1333MHz RAM stick and fill up both slots.
Already purchased another 4 GB RAM