The Asus Rampage on the X48 chipset is basically the same board as the Asus Maximus on the X38 chipset. In fact over at xtremesystems.com they've been able to flash a Rampage Bios in a Maximus board and have
all functionality. Overclocking is pretty much on par as well.
X48 seems pretty skippable to me.
The Maximus Formula Thread - Help/OCing/Guide/Rampage conversion - XtremeSystems Forums