"Since high-speed memory only provides a marginal performance benefit to a small number of all the applications on your PC, we can only recommend the step up to a subset of enthusiasts willing to pay that extra money. At the end of the day, you will need to be the judge of whether or not the added expense is worth the marginal performance increase. Regardless, this was still a review of four high-performance memory kits, and G.Skill produced the best purchasable kit in this test. Therefore its Ripjaws X DDR3-2133 CAS 9 gets our stamp of approval. "
And your MB don't support the 2400MHz but up to 2133MHZ, that means your RAM is underclock, but the 1866MHz need OC to the 2133MHZ. Most time underclock will be more stable.