Psu for $30-40 but 500watt!

DO NOT buy a cheap psu.

A cheap psu will rarely deliver the amps needed, regardless of the wattage claimed.
A cheap psu will advertise peak watts, not continuous.
A cheap psu will advertise wattage at 20c, room temperature, not 40c which will be the temperature in the case.
A cheap psu that fails can destroy any of the components it is connected to.

A Psu is the last place you should economize.
My short list of quality psu's would include Antec, Corsair, XFX, Seasonic, and PC P&C to name a few.

If a quality psu has a 6 pin pci-e connector, you can be assured it has enough extra power to run the rest of your system too, regardless of it's wattage.