If the timings and ram voltage are identical, then you won't have any issues. Both sets of ram will default to 1333, unless you force the slower ram to 1600 by overclocking. Sometimes ram will overclock when mixed. If it works, you set your ram speed to 1333 manually, and raise the cpu fsb about 20% for the overclock. the slower ram may then match the faster ram. If stability is more important, then simply set the ram to 1333 and forget the overclocking.