Your cpu is plenty fast enough, regardless of how fast the ram is.
Slower ram may look bad in synthetic benchmarks, but there is little difference in real application performance or FPS. I think the number is 1-3%.
Gaming performance is almost totally dependent on the graphics card anyway.
More than 4gb of ram won't help games much, unless you are also multitasking. Here is a study from Corsair:http://www.corsair.com/_appnotes/AN902_8GB_or_More_of_System_RAM.pdf
Considering that you already own the extra ram, I think you should use it.