The recommended is 500 watt PSU with two 6 pin PCI Express connectors now if you have a good 500 watt PSU with atleast 36 Amps on the 12 volt rail you will be ok but you will need to buy an adapter to create another 6 pin connector.
You can't use two 4-pin connectors that are sitting on the same line (rail) that comes out of the PSU for the 6-pin connector, if the line is less than 36A.