So this is what CPUZ tells me i have in computer. Which frequency is the right one. I want to buy some more RAM but I have no idea at what frequency to look.
Your memory is running at 2 x 666.7 MHz = 1333 MHz.
666.7 MHz is the frequency of the clock signal being sent to the memory. Since the memory is Double Data Rate, memory operates at two times the memory clock frequency.
DDR3 Ram's minimum frequency is 1066Mhz, 1333Mhz and 1600Mhz and some are also above 1600Mhz.
Check Ram frequency from bios, it will show you exact frequency. For checking RAM's frequency, I will recommend to check it from bios rather than CPUZ.
Look under the SPD tab and there will most of the times be a partnr.Maybe search by that.
Telling what others would do,mixing ram can give problems,even the same sticks when from a different batch so keep that in mind.
Maybe sell these old ones and buy the more ram in a single package.
It might also work fine just to let you know.