Typically, and I am an owner of an upgraded MacPro 4,1, all it takes to do said upgrade is to update the firmware on the 4,1 motherboard. So no swap out of hardware is necessary since the motherboards are the same.
The answer to the memory question is yes, you can run 1333 MHz memory in your now upgraded to 5,1 MacPro. All the installed memory has to be 1333 for it to run at that speed.
Rick
molṑn labe'
"I am a mortal enemy to arbitrary government and unlimited power. I am naturally very jealous for the rights and liberties of my country, and the least encroachment of those invaluable privileges is apt to make my blood boil."
--Ben Franklin