I see what's going on now, the video card uses an 8-pin pci-e. While the adaptor you got with it converts two 6-pin pci-e to one 8 pin pci-e.
So, what you really need is an adaptor that converts two molex 4-pin to a 8-pin pci-e.
Newegg.com - 1ST PC CORP. CB-PCI-X2 Adapter
Before you go ordering the cable, could you check the lable on your psu and tell us on many amps is on the 12volt rail of the psu. Even though you have 550 watts, the amps might not be good enough as you say it's a generic psu.
Just don't want you to waste your time and money ordering this cable when you might actually need a new psu to power it.
