taken from http://www.bit-tech.net/hardware/cpus/2011/01/07/how-to-overclock-the-intel-core-i5-2500k/2
"Memory Voltage: As with LGA1366 and LGA1156 CPUs, keep this value to within 0.5V of the VCCIO voltage to prevent long term damage to the CPU. By default, this is 1.1V, which means the 1.65V used by previous Intel DDR3 memory is still acceptable. However, more recent memory will be rated at 1.5V (or even 1.35V if you choose a low-voltage kit). Increasing the VCCIO voltage obviously gives you more overhead on your memory voltage (remember, add +0.5V at most or risk damaging your CPU)."