Buy it here.
Your PC stores data in--and retrieves data from--the hard drive in a predetermined order. Meaning: the process is mega slow, at least compared to the speeds of random access memory. Think of RAM as the temporary storage device where your PC stores data that it's using at that exact moment in time--video game textures, that sort of thing. The more you have, the better your gaming experience, because if your PC had to retrieve all game data from the hard drive alone, you wouldn't have much fun.
Unless you're hardcore into overclocking, you can pretty safely ignore RAM timings when selecting your parts. You will, however, want to pay attention to speed (the recommended setup runs at 1600MHz, which is great for gaming purposes) and capacity. 8GB is pretty much the standard these days, gaming rig or no. Keep in mind most motherboards don't support more than 32GB of RAM, and you'll definitely want to buy it in pairs to take advantage of a dual-channel (or triple, depending) configuration, which provides a nice speed boost to your system.
Budget build alternative: 2x 4GB Kingston HyperX DDR3/1600 ($80). Yep, same as the recommended build. Cut it down to 4GB if you're strapped for cash, but 8GB is the norm, even for budget PCs.