Much like CPUs, what voltage and clock speed will be stable for your RAM depends entirely on your RAM (I guess when you boost RAM clock speed a lot you need to bump the CPU memory controller's voltage too). Since it's already 2133MHz DDR3, there's no point overclocking it. Aside from that shiny high clock speed number that you'll look at once in 3 months at best after you're done, you'll get no performance increase anywhere.