Assuming you aren't using integrated, speed doesn't really matter. 8 or 12 GB won't likely mean much either. Games just don't use a ton of RAM right now. More people with 64bit OSes will change that. I would go with the 12GB, but know you won't see a change in FPS from it.
But what about FPS wise ? wich would give me the best results ? Or does it not change very much according to diffrent ram speeds and ram sizes ?
I think that 8gb of 1600 would be better because I think your motherboard supports dual channel memory. I'm not sure if 2 + 1 is the best setup, but maybe I'm mistaken. 1600 is also a bit faster, but not by much. You won't even see the difference unless you do benchmarks. Having more gbs of ram just makes your rig more "future" proof (not by a lot considering how fast technology advances), I don't think it actually improves fps. Unless you do a lot of video editing, the extra ram won't do you good and sometimes may slow you down. If you really want your computer to last a bit longer, get 16gb. But I still recommend 8gbs of 1600mhz because you could always upgrade later on. Also, what's the CAS latency of the ram, that may affect it too. I see that you've picked a 1x8 instead of a 2x4, usually single sticks have a higher CAS latency than dual channels.