Will my old RAM run with a new set?

[WiteBeam]

CPU- Intel i5 750
Ram- 4GB (2GBx2) G.SKILL Ripjaws 1600
Motherboard- ASUS P7P55D
Video card- ATI Radeon HD 4650 1 GB DDR2 Memory
OS- Windows 7 Home Premium (64-bit Edition)

This is the RAM I have now. http://www.newegg.com/Product/Product.aspx?Item=N82E16820231193
My question is, if I get a set of 2x4GB can I just use all 4 chips to make a total of 12GB of RAM?
This is the set I was looking at pairing up with or just replacing my older set. http://www.newegg.com/Product/Product.aspx?Item=N82E16820231314

Also, the pair I have now is DDR3-1600. Is my motherboard running them as 1333? I don't completely understand the diff. between 1333 and 1600.

Thanks for any help and advice.

[Goku]

Should be fine as long as the timings and voltages are properly set is all. Use a program like CPU-Z to see what its running at, should be 1600MHz if your mobo detected it properly I think. DDR3 1333 verse 1600 is the ram just running at a higher speed. Most likely you won't notice the difference FPS wise.

[WiteBeam]

Ok. Thanks Goku.