Well, If you do have more than 1 HDD and DVDrom. That should be enough. I believe GT 640 does not require any additional power and is actually very power efficient at 28nm design.
But since PSU capacitors degrade in time, ( about 5 percent every year.. depending on amount time used) I would suggest you get a new PSU instead. You can cheap ones like HEC Cougar at about $60