i have alienware x51 r2 and i wanted to update the case and psu will i need a new motherboard

well  you can  transfer everything to a new tower..  board/cpu/ram/video/hddd's and  such but you will need to remove the video card riser adaptor board and leave the power board inside the X51 case..  other then that, shouldn't be much of an  issue... 

at this point why would you even want to keep the Alienware board, its a crippled  limited board,   I would  be swaping out the board anyway for  a much  better for   the price anyway ,,