wbegg:
I'm using an old computer supply with both 12 and 5 volts with a common ground.
In that case, if you can't design the pcb with a ground plane, then I suggest you put a separate ground pin on your pcb for the PSU input, connected direct to that mosfet/capacitor junction. You can use the ground pin you already have for connecting the ground sides of input devices and the LCD. What you don't want is high switched currents from the psu to the motor going along a long pcb trace with devices connected to different points on that trace.