Memory speed doesn't affect performance much, even in games. You might see a boost in CPU-intensive scenarios like file compression, but in games, a 5% boost is optimistic (assuming it's a GPU bound game).
If you have that much money to spend on memory, I suggest getting an 8GB 1600 MHz CL7 or CL8 kit in 2x4GB configuration. Even 4 GB is a lot, but 8 GB is a "reasonable" overkill, and memory is cheap anyway.