First as suggested go for a kit with all sticks matched.
Secondly speed in MHz isnt the only factor, its half of it. Latency plays just as important a role here. Speeds go up, latency goes up.
(CL/(freq-in-MHZ) x 1000) = response in ns...lower is better. Use this to determine where your advantage is. Higher frequency does not always = better performance.
I get my best OC results leaving the RAM unchanged then manually over clocking the CPU, then Uncore and memory last.
There are plenty of guides here, have a look. :cool:
“Two things are infinite: the universe and human stupidity, I'm not sure about the former” ~ Albert Einstein