I'm building a system with a P7P55D-E and a core i5 which has a integrated memory controller rated at 1333mhz.
The only way to get a higher value is overclocking the cpu is that correct?
So based on that, i can't decide on two 2x2GB ram modules, both G.SKILL ECO Series 4GB.
One is 1333mhz rated memory module the other is rated at 1600mhz.
But if u look at the specifications u can see that the Timings vary a bit.
If i clock down the 1600mhz rated module will i be able to lower it's timings, making it work at 1333mhz and with better timings than the native 1333mhz module?
You are correct - with an i5 CPU, without OCing you will not get your RAM to run faster than 1333 MHz. Even if you could, outside of benchmarks the noticable difference is debatable. Could you run CL 7 16000 RAM with tighter timings if held to 1333 MHz? Maybe. And there's nothing to say you couldn't try to run CL7 1333 rated RAM with tighter timings. But in either case the difference would likely not be noticable.
Both are already very fast. The 1333 MHz RAM is $5 cheaper, and holds a slight edge. So for my money, if choosing between these two, I would go with the 1333 RAM.