Watch Dogs is being released shortly with requirements of 6GB memory minimum, 8GB recommended.
If you are building a low to mid range PC, 8GB is enough
If you are building a high range PC or want to support games release in a couple years time without buying more memory, choose 16GB.
Modern CPUs have at least two memory channels.
You should make use of these or you reduce the memory bandwidth available.
Choose a 2 x 4GB kit for 8GB memory or a 2 x 8GB kit for 16GB memory.