Your statement has a few things mixed up.
1. Do you have a pci-e x16 slot on your motherboard? It doesn't matter if it's electrically x8. It just needs to be x16 size.
http://www.hardwaresecrets.com/everything-you-need-to-know-about-the-pci-express/4/
2. Does your PSU have the proper power connectors? I think you need a 6pin and a 8pin? What connectors do you have?