The memory is rated at 1600MHz and overclocking depends on the mainboard. Our ASUS Blitz could boot with 1904MHz (476x7) but it was no way able to run stable tests. The limit is around 1800MHz, but of course you must sacrifice latency on the chipset or the kit, all depending on which board you are using:
We ran all standards test with AUTO settings, for the fastest clocks to bench we tweaked the BIOS. If you don't know what you are doing, you should avoid BIOS settings, as it may ruin your Windows installation during booting; but any so called "overclocking" board should reach 1667MHz with ease.