The
boot(8) program passes a memory map obtained from the system BIOS, it contains a list of valid memory regions that the kernel can use.
You can check the output of
machine memory and see if the map appears invalid, if so it could indicate that the BIOS is not detecting the installed memory as suggested by
Carpetsmoker.
If other OS's do properly detect the memory is may be a bug, manually overriding the detected memory is possible from the boot program.. but dangerous if some other memory mapped device is in the area.
Good luck, be sure to check for defective modules.. improperly seated modules.. or some sort of new fangled memory suppression option in the BIOS.