The reason for the extra 4 power connectors is to provide sufficient power to systems without overloading circuits. I have seen builders and even OEMs use 20 pin ATX connectors on a 24 pin board, but I don't recommend it.
I would upgrade your PSU before you add anything else to the system, especially a PCIe graphics card.
I recommend a minimum of 550W active PFC 80+ PSU. Here is a thread that is very informative. I suggest you choose a PSU from the brands and models that are listed there.
http://www.techsupportforum.com/forums/f210/power-supply-information-and-selection-192217.html
I would upgrade your PSU before you add anything else to the system, especially a PCIe graphics card.
I recommend a minimum of 550W active PFC 80+ PSU. Here is a thread that is very informative. I suggest you choose a PSU from the brands and models that are listed there.
http://www.techsupportforum.com/forums/f210/power-supply-information-and-selection-192217.html