Mixing ram sticks, what is better?

Hello people,
Well, I recently bought a new PC (6 months ago). It's a very low budget APU based gaming PC.
The thing is, when I bought it I made a really big mistake and went only with 1 stick of RAM, thinking that I would upgrade later on (at the moment I didn't really know that there might be problems when using 2 sticks of ram of different packages).
I believe I can buy the same RAM from the same seller again, and as it's a small seller the chances are that he's still selling the same group of RAM (don't know how to call it when he buys a very large package and sells one by one), so I might have a chance to get a RAM made with the same chips.
I plan on overclocking the RAM as much as I can, because that would really improve my gaming performance.

At the moment I have a single stick of 4GB 1600MT/s Gskill Ripjaws, running at 2133 10-12-11-30.

I can get the same, or I can get a stick of faster (1866 or 2133) Ripjaws X or Sniper, so only 1 of the sticks would have to be overclocked. Would this work on dual channel?

So, should I buy the same RAM, hoping that it would work correctly, or would I be better off buying a different, faster, higher end second stick? Which would improve my chances of getting a nice overclock?


Just in case, these are my specs:
A10-6800k
1x4GB Gskill Ripjaws @ 2133 10-12-11-30
MSI FM2-A75MA-P33
 
Solution
Hi anbello262
mixing different ram speeds , timings and voltages can be very problematic , for best system compatibility stick with exactly the same module ,brand , timings voltages etc