Here's some articles:
#1:
http://rog.asus.com/forum/showthread.php?25510-Rampage-IV-Extreme-RAM-Guide&country=&status#2:
http://rog.asus.com/forum/showthread.php?38018-Guide-Easy-Overclocking-with-Ivy-Bridge-E-%96-The-Lis...#3:
https://rog.asus.com/forum/showthread.php?31269-Asus-Rampage-IV-formula-4x8gb-G-Skill-Trident-X-2400...!!!!
I haven't read them all in detail, but's there should be the info you need.
This is from the "IVY BRIDGE-E EASY OVERCLOCKING GUIDE – THE LISTY/WORDY EDITION" by Raja from Asus (#2 in the list):
1) DDR3-1866 is supported as a stock speed. However, it is only supported with one DIMM per channel. If all slots are populated, then the maximum supported memory speed is DDR3-1600. Both situations are likely subject to memory timing, too. Intel usually makes this information available in their white papers.
Supported speeds are those the processor can run without needing any kind of voltage adjustment. Anything faster than this is defined as overclocking and quite obviously, may require adjustment of voltages and memory timings to work.
The idea then is to select a memory kit that suits one’s mindset. If plug-and-play operation is desirable, then it would be wise to select memory kits rated no faster than DDR3-1600 or DDR3-1866 (depending upon density).
If one is inclined to tune a system manually, then purchasing a faster memory kit is possible. Bear in mind that performance gains from running faster memory are minuscule in most applications. If budget is constrained, or you’re simply looking for good performance return for expenditure, then spending excessively on fast memory kits is ill-advised.
-Mikael
