Google is your friend http://www.asus.com/Motherboards/MAXIMUS_IV_EXTREMEZ/#specifications
4 x DIMM, Max. 32GB, DDR3 2200(O.C.)/1866(O.C.)/1600/1333 Hz Non-ECC, Un-buffered Memory
Dual Channel Memory Architecture
Supports Intel® Extreme Memory Profile (XMP)
* According to Intel® SPEC, the Max. 32GB memory capacity can be supported with DIMMs of 8GB (or above). ASUS will update QVL once the DIMMs are available on the market.
* Refer to www.asus.com or user manual for the Memory QVL (Qualified Vendors Lists).
* Due to CPU behavior, DDR3 2200/2000/1800 MHz memory module will run at DDR3 2133/1866/1600 MHz frequency as default.