Typically, CPUs have a recommended RAM speed. Any RAM speed past what the CPU recommends is considered "overclocked" when referencing it with a given CPU. Meanwhile, motherboard and RAM manufacturers have tested faster speed RAM and also have a Qualified Vendor List (QVL) for which RAM kits have been tested and approved for a given motherboard (and possibly CPU combo). You can check your motherboard manufacturers support page for more details on its QVL. Also note that many people are running RAM that is faster than their CPU recommends AND is also NOT on the QVL. There is always a chance that the faster RAM will not work however.