The power supply is needed to power the +12V 75W circuit in the PCIe x16 slot itself and the external PCIe power cable(s) for larger cards. If you want a card that gets all its power from the slot and your gaming is at 1680x1050 or lower resolution look into one of these: http://www.newegg.com/Product/ProductList.aspx?Submit=E...
If you will have multiple graphics cards, then, yes, they each need to be powered.
But typically it is done with a stronger power supply that has more leads.
Some basic graphics cards are powered only by the pci-e slot.
Others need auxiliary 6 or 8 pin pci-e power connectors.
If your psu has sufficient such connectors, then it will be strong enough to do the job.