Changing the PSU is quite straight foward, as all the plugs/connectors are made to fit only one specific socket.
The motherboard needs the wide 24-pin connector and either a 4- or 8-pin cpu-power connector (usually located in the top left of the motherboard).
Molex/sata connectors fit Harddrives and other peripherals and those plugs are fool/idiotproofed and can't be plugged in wrong.
The graphics card will need power from the connectors named something like "pci-e or pci power".
It's quite simple really - the hardest part is to route the cables and strip them down to ensure good airflow through the case.