Since RAM serves CPU, you need both reads and writes. RAM is used only after caches, but for whatever task you do, inputs to CPU are reads, meanwhile CPU outputs are writes. It's not even task specific, but more instruction specific. You really shouldn't worry about that for gaming, just buy whatever has highest clocks and lowest latency and is supported by board and it's going to be fine. BTW clocks>latency, because clocks decide how long each cycle takes, meanwhile latency only reduces latency cycle number, which is usually much smaller than task itself.