X38 sample boards are already in the labs and we can say that the X38 will be the next big overclocking thing. When it comes to memory speeds it should be able to push DDR3 memory all the way to DDR3 2000 if not even faster.
You will need high quality memory and some huge voltage values, but it can be done. P35 memory overclocking will simply warm you up and the X38 will be the real thing. It is also expected that Nvidia should have its new DDR3 chipset soon, but we know very little about it.
High quality DDR3 can go over 1600 right now, but we need some higher speeds than that.