If I read contrex correctly, he said that higher speed would only be wasted on your mother board. That is also my understanding from years ago when the choices were much simpler.
Roger did read me correctly. Let put it more simply: The motherboard contains a piece of hardware called a "memory controller". It contains a "clock". When choosing RAM for the computer, it is necessary that the RAM will run at the speed set by the clock. If the RAM is not capable of running at that speed, the computer will not work. Even if the RAM is capable of running faster than the clock speed, it will not do so. With some motherboards you can go into the BIOS and "overclock" the RAM by altering settings but it is very easy to put your computer into a non working state.
However I do not think that adding more RAM will give you a dramatic speed jump. This usually happens when a computer had too little RAM before e.g. 1 or 2 GB with Windows 7 or Vista. You already have 8 GB, more than enough for a modern operating system. More RAM will not make your processor, disk drives, or network connection run faster.
You do not get big speed jumps by throwing RAM at an already well specified computer. You get them by buying a new computer with faster everything.