I assume that by 48 you mean a 48G drive? If so then yes a Mk2 or a Mk2a or Mk1 for that matter can take a 48G drive. A Mk1 has more restrictions on the (edit:) physical size though.
In some PDAs such as the iPAQ, it is possible to stack RAM chips on top of the ones soldered to the board to achieve higher than stock levels of memory.