Whatever you want. The IMC, or Integrated Memory Controller, is on your chip, and this is the most important part as far as memory. The IMC is like the rest of the chip, in that there is some variance between chips. In all likelihood, it is similar to most chips' IMCs, 4 GHz QPI (AKA 8 GT/s QPI), will never be made stable with OC'd core clocks, if at all; this is what you would get running RAM at 2000 MHz. Therefore, if you want to OC with 2000 MHz RAM, you will just end up setting it at 1600 or 1700 MHz, but with tighter timings. What I am getting at, is that sn0man is entirely correct (for several reasons), when he says "look for a 1600 MHz kit".