you can mix CAS timings as long as the memory is supported by your motherboard.
But with DDR-3
you have to be very careful about mixing CAS Timings as you can cause your system to either not recognize the memory with the lower CAS Timing (faster rating) or to not boot at all depending on which memory sockets you put it in.stingreye
had this very problem here
with DDR3 memorystingreye
confirmed a few replies later @ 11-01-2009 at 03:52:09 AM
that this was indeed the problem after he had received a second set of new memory sticks to match the first set he had ordered.