If you're looking for an affordable power supply from a reliable vendor, EVGA has a 450W model that can be had for just $10 after mail-in-rebate. That's not quite free, but not far from it, either.
The EVGA 450 BT (100-BT-0450-K1) is on sale at Amazon for $30, down from $45. That's already a decent price, but a $20 mail-in-rebate is what makes this a steal, assuming 450W is enough for your setup.
To find out, you can plug your specs into any of several available online power supply calculators. EVGA has one, as does OuterVision, Cooler Master, and many others so you can get a second or third estimate.
This EVGA model is 80 Plus Bronze certified. It utilizes a single +12V rail (35A). As to the wattage rating, the 450W figure is based on continuous power rather than unsustainable peak power output, the latter of which is what some lower quality models use to inflate the specs.
Some online stores give us a small cut if you buy something through one of our links. Read our affiliate policy for more info.