The R720 originally supported up to 1600:[URL] The R720 currently supports 1866 (requires current BIOS):[URL] and memory controllers are "usually" pretty flexible when it comes to speeds ... slower of faster will usually work.  While 1600 might be the "supported" speed, sure, it may "accept" an 1866 module (of course, running at 1600), likewise slower memory will usually be "accepted".  However, there is a limit, and there is nothing hard and fast - no documentation (from Dell or Intel) - about how fast is too fast and how slow is too slow - it is dependent on the memory and the controller and if they are able to mutually communicate at a common speed (although the R720 is documented to support 800-1600 DIMMs in the manual, 800 is not mentioned in the technical documentation that documents the addition of 1866 support, so they may/may not still work).  The latest servers (DDR3) also seem to be more lenient with speed (although speed can be reduced in certain configurations) - previous servers using DDR2 would not usually tolerate more than a step - sometimes two - faster or slower than the spec'd speed.  
If DDR3 1333mhz ECC Unbuffered is accepted in a server system, does that mean different speeds of unbuffered will be accepted as well? I know if it a lesser cpu a 1333 will run at 800 or 1066.  But will a 1066 module be accepted?
Memory Compatibility