My PC turns its self off after 20mins?

Sounds like a heat issue possibly if it powers up and then crashes. Could also be that you need a more powerful PSU for that video card. Does the PSU have 2 6 pin pci express plugs or are you using molex adapters for them? if you're using the adapters try to attach each molex plug to a different wire on the PSU to try to get the power from separate power rails. Even if you have the dual 6 pin connectors that is a pretty inefficiant CPU especially to be running with an inefficient GPU, not to mention that your generic power supply likely isn't even as powerful as advertised. I'd maybe look at 600-700W name brand PSU like from antec or corsair.