memory - What is the difference in RAM speeds?

I am building a new PC with the intel i7-930 processor. I am wanting to use 12gb of RAM for it (6x2gb sticks).

Here is some of the RAM I am looking at G.SKILL 6GB (3 x 2GB) 240-Pin DDR3 SDRAM DDR3 1600 (PC3 12800) Triple Channel Kit Desktop 2 of these 3 stick kits.

They are rated at 1600 however some of the user reviews on that product say they xcould not get it to run at 1600 and the supplier who makes the RAM made a comment that "I7 doesn't support over DDR3 1066"

So I am curious, I do not know much about this, I know the more GB of RAM the better but as for the speeds, I am not sure how much of a difference it makes.

So can someone explain to me what the difference in performance may be based on speeds of 1200 vs 1600 ram?