Hello,
I'm not sure if it's a good idea to upgrade to DDR3, but if you're sure the motherboard can handle it, I'd go with the corsair vengeance 2x8GB, 1600Mhz not too expensive, good size. Or if you want to stick with kingston, kingston value ram 2x8GO 1600Mhz, either way it's affordable, and I guess the motherboard can handle a bit higher frequency than what you have now with your 2x4.

I hope this helps you in your choice.

Good day

Hello jéjé, and thank you very much for your answers.
My current motherboard is a :
GIGABYTE
If I understand these characteristics correctly, I am limited to 16GB and the frequency is 1333Mhz.
I ordered, just before your answer, 2x8GB Corsair XMS3 (2x, 8GB, DDR3-1333, DIMM 240).
I hope I'm not wrong.
I'm talking about 1600, if I opt for 1600 I'll be limited to 1333Mhz, right?
Thank you very much and have a nice evening.