The X48 is an improved version of the X38 with faster memory speeds and support for 1600MHz FSB processors:
It is still not certain if Intel will go for FSB 1600 anytime soon, but as X48 will be the faster version of X38 it looks like it will support even FSB 1600 natively.The X48 chipset will be available in November - a month after the X38 launch.
The memory overclocking will be great and with some super high class modules such as DDR3 1866 you should even go over DDR3 2000 if not even faster.