Here's a good discussion about single vs dual channel RAM: http://www.gamersnexus.net/guides/1349-ram-how-dual-channel-works-vs-single-channel. For gaming and general PC work, it doesn't matter. If you're doing serious number crunching, then the dual ram config will be beneficial. So, it comes down to price and ram frequency. For your mobo, make sure to get DDR4 2133 MHz memory. That the max frequency for this mobo. 1x8 or 2x4 probably doesn't matter. Whichever is the better price.