Switching a Large Current with a Tiny Switch
Switching ground is a Bad Idea™. Ground is your reference, and if you need one solid, reliable net in your circuit it should be ground. Don't mess with it.  

So I would move the FET to the 7805's input. Almost any P-MOSFET with a low \$R_{DS(ON)}\$ will do, like for instance the Si2323DS. Don't forget to add capacitors to input and output of the 7805, like the datasheet suggests.  

edit
Russell already drew your attention to 4 W power dissipation in the voltage regulator. Especially for SMT this may require special measures. Russell mentions a D-PAK, which has a large tab for a good thermal contact with the PCB. Remember, for these packages thermal resistance for conduction (to the PCB) is much less than for convection (to the air). You could use a heatsink for a D-PAK  

  

which mounts over the package, not onto it. You can also use a regulator in a TO-263-5 package, which has more pins to drain the heat.  

  

But the best solution remains a switching regulator, which will reduce the dissipated heat to less than half a watt.